Re: I'm ashamed...I cheated!
I think the rule of thumb is that if there is enough wind to make headway - or the expectation of that wind arriving soon - it is always more pleasant to sail. But the reality is occasionally you get becalmed; or the wind is so light it would take you an hour to get to your slip, instead of 10 minutes, and it is getting dark; or similar situations, then use the motor.
But really sailing is for FUN. Enjoy your boat, do what you enjoy - if that means motoring more than others, fine. If it means taking longer than everybody to drift through a windless channel - fine. Be true to yourself and your boat, and all will be well...