Uwe, I've wondered this too. The last time I had to regularly stop and turn material I was sewing an "infiniti" pattern on a gunbelt using my Pfaff 335. Every now and then I would get a dropped stitch and couldn't figure out why (it was always at the turning point). I've been pondering this for quite a while, trying to visualise the hook/needle operation when stopping. The only answer I could come up with is that sometimes the needle may have been on the upstroke and turning the material twisted the thread and prevented the loop forming properly. With this in mind I figured it's better to stop right at the bottom of the downstroke, before the loop can form (although stopping on the downstroke, as you say, also makes sense). Don't know if I'm right, but it sort of makes sense to me.
I shall await, with interest, comments from the more knowledgeable here.