Not currently at WDW, but
Disneyland has been talking about penalizing park reservation no-shows, apparently by either a no-show fee or restricting the nonshowing guest's ability to make more reservations. They (Disneyland) seem to be enforcing penalties against Magic Key APs who no-show to three park reservations within 90 days. These guests will be blocked from booking new park reservations for 30 days.
However, you should cancel the reservation if you know you are not going to use it, even the same day.