As a Sales Manager I think sandbagging is a waste of time but I don't fault my reps for doing it. There are times when I will address the problem like when the rep is way behind plan YTD. If you are behind plan then you don't have the luxury of playing games. Year end will also bring light and heat on sandbagging if the team is down.
I would love to have a comp plan that paid out deal by deal and we didn't close books every month, have a rolling total and quota through out the year.