We've had it both ways.
It helps to keep in mind that you do not actually have a reservation at all, at least not in the conventional sense. They are not holding a table for you. They are holding a place in line for you, starting at the time indicated. If you want to minimize waiting, you should heed the earlier advice and be 15 minutes early of your ADR time, when they will let you check in and claim your place in line. If this doesn't coincide with the reality of your travel, call and request a later time.
In most cases, when you have not checked in within 15 minutes or so after your ADR time, they have no qualms about releasing your place in line. This, as mentioned by another earlier person, is natural since you did not honor your part of the agreement - to arrive at or very close to the time you requested.
Will they give you a place in line when you show up later? Maybe yes, maybe no. It depends entirely on what is happening there/who is making these decisions on the night in question. If the restaurant has 'closed' to standby diners, it is entirely possible they will turn you away. If they are having a slow-ish night and are taking standbys, they may give you a place in line at the time when you arrive.
Remember that, especially in busy seasons, they overbook ADRs by as much as 25 or 30 percent to compensate for the fact that some folks will opt to not show up.
Also note, if you are making 4pms to try and get in on the lunch menu, if you arrive at 5 and you do get in they will definitely hand you the dinner menu...