I'd also like to hear a rational explantation of the pre-vs-post argument.
I've used preflashing to GREAT effect (in terms of the change in the image, rather than the images being particularly great...). It's something that should be taught as standard practice. If you haven't tried it then do so at once. It does allow you to move the highlights without touching the mids or blacks signifigantly. As such it's an extra tool that does something you can't do with exposure and contrast alone.
I've heard several times the pre-vs-post argument, but if the halides just counts photons that hit it, then why do they care if they're hit by flash then print or print then flash light? photons is photons. 3 flash + 2 print = 5 photons which ever way you add them up.
The only explanation which MIGHT ring true is that the flash pushes them to a critical level (which is the point). Some highlights might receive no further light from the main exposure, and the halides recombine by the time the print hits the dev (after a longer time than post flash). Pre-exposure would therefore affect the mids more (where they receive more additional exposure), but produce less fogging in the real highlights, as these don't develop at all. However - this is a MIGHT, MAYBE, if I HAD TO explain it kind of explanation - I don't actually believe it myself. If it were true you could "save" accidently exposed paper by putting it back in the dark for a year or so.
I do know someone to ask though... I'll ask him next time i see him...