4x5 in 2500 series tank and 120 in 1500 series tank: I use a prewash to prevent surge marks along the film edges, for both color and b&w. I get uneven development without a prewash with these combinations. For 135 it doesn't matter and either prewash or not works OK. Development times for b&w aren't all that critical in my book so either go with the official time or deduct 10 or 15% or whatever, really. For color (C41) I just go with the official time.
The important thing is to establish what works for you; as you can tell from the varied responses in this thread there's no consensus and there's never going to be one, either. So read the posts for your amusement, then ignore them and TEST what works for you.
Have fun with the machine!