The reason you have gears to shift in the first place is to maintain the proper RPM for optimum performance of your engine. That applies to acceleration as well as braking.
I always downshift prior to applying the brakes. You will be able to maintain better control of your vehicle, and you will be in a better position to accelarate sooner if and when the situation calls for it. The only exception to this, is if some bonehead does something that forces me to brake first, and then pick the correct gear.
There is nothing wrong with correctly downshifting to aid in braking, or even in lieu of braking, if you do it correctly. This applies to manual as well as automatic transmissions. And there should be no excessive wear caused by correctly downshifting your car. The key is doing it smoothly, and being aware of what gear you should be in at a given speed, and them matching the RPM to those two factors. Speed+RPM+correct gear.
Next time you drive your car, make note of the RPM range you run in before shifting. What is the high RPM when you shift, and what is the low RPM after you shift (when you let out the clutch)? Then, keep track of your speed range in all of the gears (this is easier if you have someone write it all down while you are driving), For instance, 1st gear, 0 to 15 MPH, 2nd gear, 15 to 30 MPH, etc. So if you know, for instance that you are going 25 MPH in 2nd gear at 3,000 RPM, you will know that if you want to shift down to 2nd from 3rd, that you need to do so when you are going 25 MPH, and will need to increase your RPM to 3,000 in order to do a smooth shift. Then, after you do that, you can decrease throttle pressure to slow down smoothly.
Just remember this: Whatever speed you are going when you shift into a higher gear, is just a different RPM for either gear. 25 MPH in 2nd = 3,000 RPM; 25 MPH in 3rd = 1,200 RPM, get it?
Again, the purpose of shifting is to keep your engine operating in the optimim power curve for the drive train on your car. I hope you follow what it is I am saying, and understand that all of the numbers I am using here are simply hypothetical examples and not absolute guidelines of any sort.
This stuff takes time and practice to get it right, and high performance driving schools actually charge you money to learn how to do this correctly.
[Edited by longston on 03-03-2001 at 07:32 PM]