Disney suggests that you arrive a little early, between 5 - 15 minutes, before your dining reservation time. If the 8 AM reservation is an in park character meal, many want to arrive early so that they have time to take photos of a nearly empty park (esp. MK).
As far as arriving late, there is no set policy on how long a restaurant will hold your reservation before considering you a no show. Most will give a 15 minute grace period, but we were at a restaurant which was running far behind schedule so they were turning away latecomers and not allowing a grace period. I would make every effort to be on time and not depend on the good will of the restaurant