Everybody has to make their own decisions on that; I say yes it is clearly best to have them pop tested when replacing the Spray Nozzle (and also to lap the internal precision surfaces before assembly).
Maybe I can explain it a little better.
There are a couple of things that happen as injectors are used. 1- The Pintel seating area in the spray nozzle wears out and they start to dribble raw/un-atomized fuel before their set pop/opening pressure is reached. 2- The tension in the spring inside of the injectors tends to weaken with use and when tested the pop/opening pressure is after many years of use lower than spec and they are not balanced between the set of injectors.
If you know the seat in the injector Spray Nozzle the is no good (you test for this on the pop tester). You will get improvement just by changing the Spray Nozzle (Diesel Giant has a how to do; do one nozzle at a time to keep the parts together for that nozzle). But the dimensions of the new Spray Nozzle may not be the same as the old one (especially Monark vs. Bosch) and this will change the tension on the spring in the injector. Meaning that the Pop/opening pressure of the nozzle will change and they will not be balanced between the other injectors (your injectors all could have different pop pressures; maybe out of spec; no way to tell without pop testing).
The pop/opening pressure of the nozzle also has an effect on your injection timing. Lower pressure injects the fuel sooner than a higher pressure.

Depending on my fanancial situation at the time I could see myself only changing the Spray Nozzle with out pop testing if my car was running poorly.
In my case when I bought a diesel car (Volvo) in 1992 I knew the injectors neededrebuilding and I bought a cheap ($150)Chinese Pop Tester right from the start. I figured a good part of the purchasecost of the pop tester would would be saved by not having to have 6 injectors rebuilt in a shop.

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84 300D, 82 Volvo 244Gl Diesel

Thats a pretty nice unit that he came up with over at **************.....I may consider one in the future..... it practically costs that much to have a shop do a set or two of injectors....so why not have your own!

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-diesel is not just a fuel, its a way of life-
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'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

Last week one of the members posted how he/she made one from a jack similar to the mercedes source one.
I found the thread: http://www.peachparts.com/shopforum/showthread.php?t=224334&highlight=pop+tester
His homemade pop tester I believe is better than the ************** one as I do not think the ************** pop tester had a valve that cuts off the gauge.Though this valve is not 100% needed it protects the gauge when you want to do some serious pumping on the tester handle.
Perhaps he could be prevailed upon to produce some more for sale

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84 300D, 82 Volvo 244Gl Diesel

So, if timing has changed, how do you time it properly? I've got an AB light and set it to 13 ATDC. How do you actually know when the pump is opening the injector to set it right?

I am not sure I understand the above comment.

"So, if timing has changed, how do you time it properly? I've got an AB light and set it to 13 ATDC."
I have not used the AB light or ever completely read the instructions. However, if you follow the FSM instructions your timing should be correct.

If the timing is not correct you have to loosen all of the IP mounting bolts and I also recommend loosening the Hard Line Nuts at both ends (so you can rotate the pump easily) and rotate the IP and recheck it.
If you have a newer IP that has a screw to rotate the IP follow the FSM directrections.

"How do you actually know when the pump is opening the injector to set it right?"
If the Injector opening pressures and the IP Begin Injection/Timing are both with in specs ignition should happen when it is supposed to.

It is sort of a prediction that if the Injectors are set within in a certain Opening/Pop Pressure range and the IP begin Injection is set with in the specified range of degrees that Ignition will occur at the proper time.

I believe the AB Light uses a spot on the IP Camshaft to trigger the Light and because that spot is not in the same place as the begin Injection of #1 if you used the Drip Method the Light triggers and a differnt degree mark. Mercedes has calculated which degree/s that is and gives you a different degree spec to compensate for that.

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84 300D, 82 Volvo 244Gl Diesel

You had mentioned that the timing would change with the pop pressure. I had wondered if that changed, how you would adjust.

Yes, the IP light uses a spot on the IP camshaft. This was recommended by a Mercedes mechanic to me.

I am not sure what the FSM is or how to use the drip method. Is the drip method better than the AB light?

The Pop/Nozzle opening pressure is adjusted in each Injector by changing the Shim/s that tension the Injector spring. You need a Nozzle/Pop Tester to do this (you can do a search on Ebay to see a pic). It is a hand pumped hydraulic tester that has a gauge to measure the Pop Pressure.
Changing the spring tension changes the Pop Pressure.

The FSM = Factory Service Manual. There is a description in the DIY section and maybe instructions on how to use the AB light.

I have no experience using the AB light but it does not require that partly take out the #1 Delivery Valve and Delivery Valve spring (Depending on the Model of IP you might need a special Splined Socket to remove and install the Delivery Valve Holder) and you would also need new sealing parts (crush washer and O-ring depending on the model of IP) and a Torque Wrench.

So I suppose the above alone could make the AB light easier to use.

Some members have had frustrating experiences with the drip timing method (best to go to the DIY section and read the instructions to see for yourself).

In the past more that 90% of all inline type Fuel Injection Pumps used some form of the Drip Time Method to time them to the Engine so it clearly works. But, you have to understand what you are doing.

When I first did it 30+ years ago I had my Boss to guide me through it till I got it right. After that while it was not something I looked forward to it was not a problem to Drip Time an IP.

After not working as a Diesel Mechanic since 1990 or so I had to re-learn how to do the Drip Time again now that I have a Mercedes (with the Volvo you have to use a Dial Indicator setup).

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84 300D, 82 Volvo 244Gl Diesel