I'd talk to the teacher either way, if it was good or bad. /righteous
Although, I think most people (in some cases, myself included) would just shrug off the 'bonus points' as something nice that shouldn't be trifled with. If it happened more than once, I would definitely talk to the teacher. If it was something major, I would ask the teacher. My Algebra teacher marked my midterm exam up to a 79 from a 71 - huge difference, and I didn't know until I got the exam back 2 weeks later. I talked to him about it because it just wasn't right. He didn't understand his mistake, which was confusing and frustrating (especially because it was blatantly obvious - clearly got a 5/18 on this one section and he added it up to be 14/18, that's embarrassing). So sometimes it's easier to just let it slide, especially after the fact.