Funny this topic coming up now. I use pre-flashing for my paper negatives to help control contrast a little and increase speed. I learned about it by reading Joe VanCleave's notes over at f295.org but I know he's posted his method here at APUG as well.
I make a test strip for each paper type, and pick the exposure that is 1 second shorter than the faintest visible tone in good light after dry-down. Joe uses graded paper but I use VC paper and only preflash through a green filter, so I'm differentially pre-flashing the low contrast emulsion.
The reason this post was timely is that recently I've made a number of paper negatives using post flashing instead. This was due to being in a hurry or wanting to reload a one-shot camera while a paper was developing and not wanting to turn on my enlarger during that time. Using exactly the same enlarger height/aperture and filters, for exactly the same amount of time that I use for pre-flashing, I am unable to see any difference. So in my context, post-flashing can be more convenient... pull the paper out of the camera, stick it under the enlarger for 17 seconds ( or whatever for the paper type ) start my pre-soak and load the camera with a fresh sheet of paper. I'm almost sure I remember Joe saying that post-flashing was almost the same as pre-flashing, but not exactly the same. But in practical terms, my experience says I can use them interchangeably. Whatever the difference is, it's too small to matter for my mostly pinhole photos. I also use paper in lensed cameras, and have post-flashed a few of those in the past few months too, with equally acceptable results.
I have never used pre or post-flashing during printing, so I can't comment on that at all.