I have used both sorts. I currently use standard 'self tappers' into acetal and carefully calibrate the size of the drill bit for the hole.
This was for some very small self-tappers (#0). I drilled two different sized holes, inserted the STs, and then milled the block in half to look at the threads formed. There was not much difference in the appearance of the threads, but driving the ST into the 1.2 mm hole was MUCH harder than into the 1.3 mm hole. So for production we use the 1.3 mm size. So far, no problems.
Cheers
Roger