Why does a wife beater (a-shirt?) help with CCW? So it isn't rubbing your skin all day?
Both sorts of undershirts have their purpose.
T-undershirts are great when worn under polos, button-downs, and heavier sorts of shirts. Suck up sweat all over, prevent chafing from IWB holsters, bits of the heater that may not be covered by the holster, and your mag & mag carrier.
A-shirts are great when you want to CCW under an untucked T-shirt. They serve a similar purpose as the T-undershirt, but are used when you don't want the undershirt visible.BTW, the untucked T-shirt is best made of heavier material, a darker color, and perhaps a pattern.
Another use for the A-shirt is when you CCW IWB commando--no under
shorts. Make sure you buy the A-shirt extra-long. The Wal-mart FotL A-shirts have an unhemmed bottom and are longer than similarly-sized A-shirts from Hanes, etc. The A-shirt then prevents chafing up high as well as down low, where your undershorts would be.