I recently read a post which reminded me of a very effective rule I adopted in some of my hotels:
If the answer to a guest's question is "no", you need to find me or your supervisor first. On the other hand, if the answer is "yes", go for it....make the guest happy.
99 times out of a 100 this will not lead to a "giving away the house" scenario. And if it does, who cares, it's only 1 out of a 100.
I think this rule is essential to providing Memorable Service, and creating an emotional bond with your customer/guest.