As a courtesy to others it's always a good idea to cancel any ADRs you can't keep. The only restaurants that penalize you are the ones that take a credit card at reservation. Places like CRT, CG, etc.
I know when I make our ADRs that some of them won't stick. Our plans for that day may change (not going into parks the day we have a park restaurant scheduled, etc.) As soon as I know I can't make it I cancel it, usually at the same time I'm making a new one.
By cancelling your ADR you're giving another family a chance to make their own ADR.
Course, you mentioned you might not be able to make it if your flight's delayed. Will you be able to call and cancel?