Pre-wash. There has been a lot of debate, but much of it is not grounded on any chemically rational discussion. For color development, pre-wash warmed the film, reels and tank to the operating temperature, and for that reason alone, pre-wash becomes standard operating procedure. B&W development makes pre-wash much harder to justify, to the extent that justification is required. Temperature control of the tank system is rarely required, but that remains valid where needed. Arguments made for even infusion of developer are made but not supported by any demonstration of truth, and it makes the assumption that developer will displace water in the emulsion more evenly than a dry emulsion will absorb it, which is ridiculous on its face. There is the old argument that pre-wash needlessly extends "wet time" for the whole development process, which is deemed bad. That may be the case with old, thick emulsion sheet film, but is unlikely to be a relevant concern for modern, thin emulsions. Pre-wash will remove and discard anti-halation dye in the emulsion, which is meaningless to the development process itself. If you use one-shot developer, pre-wash is obviously a waste of time for that purpose, as the developer is discarded after use. For those who reuse their developer, such dye accumulation can be emotionally disquieting, but that is a "you problem". For certain types of developer, pre-wash can be chemically disruptive. An example would be a divided developer, which is dependent on the amount of "first bath" being absorbed into the emulsion to regulate the development process. Having to displace pre-wash water from the emulsion can only risk uneven development overall. My personal evaluation is that I use a pre-wash for all color processing to regulate initial temperature. For B&W, I always use a one-shot developer to maintain consistency, and I never pre-wash. I've got better uses for my time.