Everyone pretty much hit the nail on the head. Once it fits through the TSA machine, you pretty much won't have to check it in. However, it must fit in the size wize stand at the gates in order to bring it on board(I know for sure with Delta). If it doesn't fit, they would
A. put a valet tag on it and bring it back to the gate(ie with strollers/wheelchairs).
B. check it in at the gate(don't really like to do this as it takes up time.

My suggestion, don't over pack and use the smallest bag possible to fit everything. Also, don't show the gate agent and try to board last that way, it will be late to think of checking it in at the gate.(option A would be most likely). And if they fuss about it, just apologize and don't argue back. Abd if you do check your bag but you don't get on the flight, by all means let it go. When you get there, baggage claim will be open. Just keep a couple days of under clothes/shirts and bare necessities with your person.

Disclaimer: this coming from a gate/ramp agent and a frequent traveler may is not guaranteed but likely. By all means if you can avoid checking your bags, do just that.