Sprint will not require a contract for postpaid service if you pay $10 per month over and above the normal plan fees. You can even switch from being in contract to doing the $10 per month (but you will have to pay the $150 early term fee for that privilege). Of course, they don't charge you extra for month-to-month if you've fulfilled your contractual obligation.
Back when my wife got her Sprint phone in late 2000, their in-store local coverage map didn't indicate that our neighborhood was covered -- which is odd, because they have a tower a mile or so from our house -- so we went month-to-month for a couple months until we were satisfied that coverage was what we needed, after which we signed a contract and eliminated the $10 fee.
JustThe.net - Apple Valley, CA -
"The wisdom of a fool won't set you free" --New Order, "Bizarre Love Triangle"