the government has spent a lot of money to get us appointments, and to trust you enough to give you one- they know you must be a good student. This being said, the chance of losing your appointment to my understanding is very low unless you show a --pattern-- of decline in many areas; one rough class won't eliminate you. The idea is to be reasonable, not just completely drop the ball.
I've approached it as a balance: experience senior year and enjoy the last bit of free time I'll have for 10 years--- but also have enough pride and be within reason, to get at least B's in class.