I used to do the full mag +1, but I don't top off anymore for three reasons:
1. I don't like having to squeeze that mag in there so tightly. My 457 came with the plastic mag release tab, and I upgraded it to the metal one, but still...I'm not fond of that much extra pressure on the catch. My 908 already had the metal one in it when I got it. My GLOCK 27 was a BEAR to top off. Man, I just about had to put that bugger in a vise to top it off until the mags broke in. And even then it wasn't easy. I've had mags drop out when I thought they were seated, too. I wouldn't want that to happen when I needed more than one "boom."
2. I seem to have had more feed malfunctions in ALL semis (not just S&W's) when I top off, as opposed to just filling the gun with 1 full magazine worth of cartridges. Not that it has happened often...but often enough to give me pause. I'd rather have 7 at 99% than 8 at 95%. But that's just me.
3. This may sound odd, but I have found it to be one more tier of safety. If I make it a practice to not have more cartridges out than will fit in the mag(s), I am further reassured that there are none in the chamber when the mag(s) I have out are out are visably full. This is OBVIOUSLY not part of standard gun safety procedures and should not be used by anyone as such. It's just an ADDITIONAL visual aid.
Topping off is a subjective thing though, so to each their own. What I do works for me. Good topic, though. Thanks!