I disputed a grade once or thrice...
a) Was kicked out of a calculus prof's office while defending a proof. Bruised his ego I guess...
b) Confused a T.A. in UI design who had a habit of assigning low grades with no comments. period. The final grade distribution was like a camel's back (one hump @ 65%, one @ 85%). All his marked papers were at the first and the other T.A.'s were at the second (lucky I insisted on switching).
c) Got into an argument with another T.A. in SFU Kinesiology over the predominant source of physiological traits. I claimed it was mainly environmental with specific genetic characteristics passed from generation to generation. She claimed it was purely genetics, "Look at African, they are all genetically superior to all race". She had a fun time pulling her foot out of her mouth...I prevailed on the mark.

So my advice is this: don't berate the prof, they're in charge. You might have to bite the bullet and tell them what they want to hear every now and then. Be wary of who marks your paper, not all T.A.'s are experts in their field. I've dealt with enough in applied science at SFU to assure you of that.