Don't Set Yourself Up for a Power Struggle
It certainly helps to sugarcoat some situations ("After we go to the supermarket, we can stop at the playground"). However, when you really need your child to do something, you could give her a choice about how or when it might get done -- but not if it should get done. Little kids (and bigger ones too) are constantly looking for ways to have more control, so it should be no surprise that if you give your child veto power, he's going to vote nay. You're setting yourself up for a potential power struggle.