You should use the 20 amp breaker if you want to use a 20 amp outlet. NEC section 210.21(B) is the cited section that says the receptacle shall have an amp rating not less than that of the circuit. You can use 15 amp outlets on a multi-outlet 20 amp circuit, but that's spelled out specifically in that code section. No problem using a 20 amp breaker with 10 gauge wire.