It was typical and legal for houses to be wired using 15 amp receptacles on a 20 amp circuit in the past, and may/probably still is in some areas. If you read the back of the 15 amp ones you'll see they are rated for a 20 amp feed through capacity. The logic is/was most appliances by themselves don't draw more than the amperage rating of the 15 amp receptacles. Having said this, as an electrician, if you know for sure the circuit is wired with 12 awg wire and a 20 amp breaker, then go spend the couple of bucks and swap out the receptacles for the 20 amp ones. Will it really matter? Probably not, but for the peace of mind, Why not?