Sometime in the next couple of weeks I'll make my 1st delivery valve seals replacement.

From reading and watching videos it seems the springs are typically not checked for length or routinely replaced.

Question: The cost is low, any reason not to replace them all with new ones from Mercedes?

How many miles? Does your diesel produce nail sound?
If it does and you had replaced fuel nozzle and still nail sound, you might try springs too. But I would do more 7k, 7 f, 7g.

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91' MB 350 SDL, with 961 engine and #14 valve head
09' ML320 Bluetec

Don't replace the delivery valves unless they actually need it. It's unlikely you do unless your engine is nailing badly and you've exhausted all other possibilities to iron it out. The parts cost alone is about halfway to a fully rebuilt IP (which would be money better spent IMHO if your IP were that worn).

Replacing the seals is common on the OM60x's and isn't a hard job. Replacing the springs is debatable, the verdict seems to be split 50/50 whether you need to or not. The FSM says to do it, so I did it on mine, I didn't notice any difference in length between the old and the new springs.

It's far more important to make sure that the working area is *SURGICALLY* clean (you literally cannot get it clean enough). When reassembling, apply some diesel to the new O-rings and do the tightening procedure - 30nm, loosen, 30nm, loosen, 30nm, loosen, 35nm. Don't overtighten, don't skip tighten/loosen steps, don't forget a bit of diesel on the O-rings.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]