In reply to beardy mike:
Well, I believe they wouldn't say "for smooth action" then. You might be right, but some screwgates even feature I-beam construction on their top bit (Mammut Mythos, Wild Country Helix, DMM Sentinel, Edelrid Strike etc.). it doesn't look like that manufacturers really care about the wear on the I-beam construction (or they do care about you buying a new belay carabiner because the former one is worn!).
I realize that I usually keep my hand on the rope (to avoid having slack) anyway so there's no huge gap between the belay plate and carabiner. If it's loaded, it will never rotate either. I also try not to do multiple things while belaying (no matter with what device), so I keep an eye at the system most of the time. Even while climbing at the wall, I glance at the gate more than once while my partner climbs a single route - I finally have an OCD which isn't harmful!
Also, I would be more concerned about loading on the gate side because of two reasons.
Firstly, running the rope over the gate bit would unscrew the screwgate (as you can insert the screwgate from the left or right, it would either loosen or tighten the gate) and you might be unfortunate to drop your partner if unlocked screwgate hits a bulge or something.
Secondly, screwgate is really weakened if it's loaded on a gate side. Unless you belay a second with a kitchen sink attached to him/her, the carabiner would still be strong enough. If I recall correctly, it's at least couple of kN which is still a lot on a system where shock loading is not likely to happen.
However, I have never heard of either situation happening. Both situations require a high level of inattentiveness and stupidity combined!