Our bonus policy is based mostly the recommendation of our office manager, who started as an MA long ago, and over time has gotten certified as an X-ray tech and lab technologist as well. Her approach is that anyone can reach any non-MD position in the office by working hard and getting extra skills that benefit the practice. When issues of pay arise, she gives the complainer a goal that will allow them to reach the next step. As Bert says, most of them don't bother, but at least the ground rules are in place.
I haven't seen any resentment or jealousy about bonuses, because we go to great lengths to make sure they are specific and targeted, the differences in $ are not vast, and no one walks away empty handed. Someone we wouldn't give a bonus to is usually asked to leave.