Since learning to develop film in 1969, I've never prewashed -- until I started processing film again this year after a dozen years away. Initially, I was just trying to avoid Fomapan blue in my Df96 monobath by washing the dye out before pouring the active chemical, but I also found evidence (spot reading back in stuff from PE, among other sources) that developer actually soaks into emulsion faster if the gelatin has already been swelled by a water soak. I haven't seen any indication it's doing any harm; for C-41, it helps to keep the tank down in the tempering bath before the developer goes in, and I can presoak the film while I measure developer, replenisher, stop, and fixer -- so it doesn't take any extra time.
One place I haven't tried it yet is two-bath developers, but if PE's statement about faster developer diffusion in wet film is correct (and I have no reason to believe otherwise), it ought to do no harm there, either. There may be a Dignan C-41 with prewash in my near future.