From my experiences, if you have a "confirmed" ticket most US Carriers do not require reconformation. If you do not have a seat assignment, however, or want to check the status of your flights, it can't hurt to call to get your seat assignment (usually starting w/in 24hrs of the flight) confirm flight times (especially if you booked well in advance as flight times often shift, if even slightly) or try for upgrades, if applicable. It's not necessary, but it can't hurt.