The number one sandbagging complaint in the APA is at tournaments. In the beginning I didn't put much thought into it but then Mike (FR) said something a while back that caught my attention (and I mentioned it earlier in this thread) - a player can/will play 50 points above/below their rating. You don't think the APA players are psyched about playing in Vegas and work their ass off? During the rest of the year they're just banging balls and having fun but if they're playing a tournament, even local, they'll practice their ass off (if they want to win).
So, is there sandbagging, yep, is it rampant - not in the APA and I doubt it's "rampant" anywhere, but IMHO, it is more likely to happen in a FR league. In the APA you're in it for the fun and a small chance at a free trip to Vegas, in FR, it's a much more personally driven goal - to win money