The only catch for staying off site is that you need to make each ADR of your trip at that 180 day mark. In other words, you can't do the "plus 10" that those staying on property get.
My advice is unless you want hard to get restaurants, wait until the last day of your trip to book. If you want hard to get restaurants (like me this year...ugh) such as Crystal Palace pre-park opening, Le Cellier, CRT, etc... you'll need to get up each day to book them.