Are we talking B&W or colour development here? Most people don't pre-wash B&W film at all, in fact some are vehement that it is a bad idea. Efke recommends it, but the big brands (Fuji, Kodak, Ilford) don't.

My FAQ recommends a pre-wash at process-temp for C41 and E6 since they're pretty temp-sensitive and a tank full of film and spirals will almost always be notably colder (probably 12-20C) than process-temp without a pre-wash, so the error in operating temperature could be significant. B&W is so close to most people's room temp (probably within 3C) that the operating-temp errors are smaller and it's also easy to correct minor contrast variations in printing B&W.

A pre-wash will change your developer activity (in a film-dependent way) slightly due to swelling of the emulsion. If you're tuning your development results for a particular contrast, you need to be consistent about always doing or not doing a pre-wash.