I've done it now and then with transparency films but had more use for it with pinhole as a way to effectively increase the speed of slow ortho film or papers, and tame the contrast. After preflash the sensitivity of ortho plus can be as high as ISO 30-40 (woohoo!), and likewise paper gets a boost up to 8-15 or so, as I recall without loking in my notes. To me that speed boost was always more important than whatever minor change I saw in contrast. Opinions vary.
Also, sometimes when wet printing, if it looks like contrast grade adjustments aren't doing the trick and split grade neither, sometimes I preflash the paper in a last ditch attempt to tame the contrast. Kind of a last resort. Recently I have learned some other methods and am working more on those.
For most film preflashes you want to go three stops faster than exposure or so, lens defocused on a featureless subject like a piece of paper.... or you can get one of the fancy plastic diffuser things you fit over your lens. I used a white plastic filter cover once and that seemed fine. However, I am pretty economical with my film so unfortunately I can't offer any shots with/without preflash. There must be some rigorous examples online though.
I have one and only one complaint about the mamiya 6, which is my favourite camera all-round. My complaint is that it has no double exposure capability and hence I can't preflash when using my favourite film, velvia 100. If I did it I'd have to preflash the whole roll and then reroll it 🙁 It's a serious enough issue with velvia that I have considered <gasp> moving to another camera body... but I still can't bring myself to it.
ed: corrected spelling here and there.
