Unless you have to give a credit card to hold the ADR, you can cancel at anytime. The ones that come to mind that require a cc are CRT, Princess in Norway, and the signature restaurants.
We had an 11:30 ADR for lunch at LeCellier last May, that was the first lunch seating. There were three families who walked in and asked if they could get in and they couldn't get in at all.
I would make the ADRs for lunch if you think you'll need one, and then cancel it if you decide you don't need it.