I enforce that one, and like it was said earlier, I also require students to submit their objections in writing and point to specific instances where they think they addressed the prompt/assignment/question and why they deserve a higher grade. If you're very serious about the objection, you'll do that and talk to me, but only a few ever do that with me. Since I've mostly TAed so far, I tell students at the beginning of the quarter that I'm very laissez faire when it comes to their participation and assignments--I'm not going to be sure that they've turned in everything that they need to, and I'm not going to ask them to attend discussion sections and participate. Still, one or two students will not turn in an assignment--even something major like an essay--and will complain to me when they get a bad grade at the end of the quarter. Or they'll never come to class and wonder why their participation grade is so bad. I say on the first day of class that they're in college and if they're old enough to vote, join the Army, and buy pornography, then they're old enough to keep track of what they have to do for class.