I set a restriction on my horse and it is on the contract that we have with the half leaser. My horse is restricted to 3'3" courses and 3'6" single and must be wearing jumping boots past 2'9" (unless being shown). He has an old racing injury and, although it has never caused any problems, we still don't feel like we should push him higher that that, plus, I never really want to go over 3'3".
My trainer restricts us to 3' on other peoples horses. We rarely go over 3' in class on our own horses anyway.