I would think so myself. But for some it may be the difference in walking away with a trophy or coming in 13th. I have only shot one sanctioned match (2011 NJ State Championships) to this point so I don't have much experience with it. But is was enough of a topic that they spent an entire episode of Power Factor on Sandbagging Grand-baggingng. I think it is probably a bigger problem in USPSA & Bullseye where there is actual prizes to be had. I know when I shot PPC it was common place to do so. Of course depending on the number of shooters in an IDPA sanctioned match the classification winner & possibly 2nd place gets bumped to the next higher class automatically. So in IDPA it could be a one shot deal.
Right now I'm shooting mainly club matches and I don't pay much attention to the classifications, I watch to see how I do in my division overall. Most of the time I look to see how I did overall. If I'm not in the top 5 overall I'm usually pissed for days. The only exception is when the Pro's come out to play, then I take it for granted I'm going to get spanked.