More is better than less, after all we're Americans (or most of us are). 10 gauge in most instances is overkill excepting for long runs from the breaker to the outlet. Overkill is always better, less resistance and heat. For a few dollars more why go with less?
The circuit is there not to protect what is plugged into it but to ensure the wire (romex) running from the breaker to the outlets (load) doesn't overheat by drawing too much current through the line, overloaded circuit, pop goes the breaker. A cord rated for 15 amps plugged into a 20 amp outlet is only under sized when the amp draws more than the cord rating.
The circuit is there not to protect what is plugged into it but to ensure the wire (romex) running from the breaker to the outlets (load) doesn't overheat by drawing too much current through the line, overloaded circuit, pop goes the breaker. A cord rated for 15 amps plugged into a 20 amp outlet is only under sized when the amp draws more than the cord rating.