No question is stupid, and this, like most things is a personal choice.
I am NOT against paying for grades, but in your case I probably would not go that way since she has always been self motivated.....
My opinion, in your case, I would NOT pay for grades. Since you have not paid her up until now and all the sudden pay for straight A's I think this could encourage being a little too anal about her grades. If she is self motivated, great! But in todays world with pressure on kids, and especially girls, I worry about teaching our kids to worry too much about perfection.
However, I think a little celebration is in order for a job well done! Why not go out to dinner in celebration and let her pick the restaurant. That's what we would do.