I don't do a prewash with B&W, but I think that the longer developer times (6minutes or more) gives the developer time to 'even out'.
Please correct me if I'm wrong:Some films like Foma and Efke require a prewash to get rid of the anti-halation layer - no? I know when I presoak Efke, bright (pretty) blue water comes out the tank. Pre-washing helps eliminate air bells. I add a couple drops of wetting agent in the prewash water.
I don't pre-wash ilford films, as their instructions are clear against it - I put the drops of wetting agent into the developer.
E.