I also disagree with Thisisit. I was nearly 100% sure I wanted to specialize before dental school and its what kept me working the whole time to stay at the top of the class. I don't think where you went to dental school will play a major roll in how post grad programs look at you, but it will play a minor one. Some schools will help you be a more competitive applicant, ie offering research opportunities-some have masters degrees you can complete during dental school (that really helps set you apart in some specialties), some give you more time to do away rotations/externships (how many you do can make a difference, and some schools are difficult to get leave from), how is the schools ratio of subjectivity/objectivity, etc.
Having said that, I don't think you have to spend a fortune...I would avoid most private schools for that reason. Don't go to the cheapest--go to the one that offers what you want/need at the lowest price...this may still cost more than the cheapest out there.