Travelling the USA with 2 mates?

Im travelling the US with 2 mates, is it cheaper to buy domestic flights when we get there or should we pre-book and as we arent sure on our destinations yet? Also the same for car rental, we are driving LA-Vegas and Miami- NYC via Washington DC. I have also heard alot about New Orleans is it worth the detour? We have planned dallas is that worth the detour??