I Flew Spirit Air last week, and they always make you pay for every little extra luxury. How ever at the check in counter, because it was a very full flight, they offered to check my carry on bag for free right then and there, they said at the gate it would have been checked anyways. So I have not tried this yet, but next time I am on any airline, and the flight it full or oversold, it may work to tell them "hey this is gonna end up being checked by the gate, so why not just check it here for me now" they can even put the tag that shows checked by gate on it then and there, so its not in the system and their supervisors would not ask why they did not charge...
just something to give it a try...If this works for anyone, let me know!!!