I agree with bigred. It all depends on the teacher. At pitt, our math curve was already set by the dept. it was like 100-90 is A/A-, 89-80 is B/B-, 79-50 is a C/C- etc
In my bio class, the teacher curved each test down/up to a 75% average.
Basically, if everyone does bad, teachers will generally curve up to a certain avg.
I dont know any profs that do a full curve (ie 10% get A, 30% B, etc). Although thre are some that have departmental rules, like 20-30% students should get a A or B, etc