As the PPs indicated, there is no hard-and-fast rule about whether you can show up early for a reservation or not. Most of the restaurants will try to accommodate you if they can, but if they are already full or running behind schedule, they may not allow you to check in early. You are asked to arrive 15 minutes early to your ADR and that's when you should be able to check in (no matter what). I have been there when venues (e.g., Sci Fi, Le Cellier, BoG, CRT, 'Ohana, etc.) were telling folks to come back 15 minutes before their ADR time even if the 15 minute mark was only a few minutes away.