I would suggest making a separate reservation for your first night if you want to stay at WL, otherwise your pass will last from the moment of check in and you will spend money on a day that you won't use.
With a separate reservation, it is always possible that you may have to switch rooms, but Disney Cast Members, being the helpful people that they are, try to keep you in the same room if at all possible. We have done this a couple of times at the Boardwalk (can't remember why we had two reservations but there was no pass issue because we have annual passes), and they have always been able to keep us in one room. But, there is never a guarantee on that.
All that said -- I would advise calling WDW and checking on the pass situation. Whether they charge you for the first day on your pass may depend on how late you arrive on that first day. Also, I have heard guests (in the interest of full disclosure: I am a
Disney Store Cast Member) telling me that they were able to work out something with Central Reservations in which their passes did not "start" until the second day. I have not experienced this personally, so I don't know how it was done, but it is worth a call to WDW to check the possibility out.
Enjoy WL -- it is a fabulous resort, one of our favorites. If you have the chance, eat at Artist Point. It is one of the best restaurants on property -- food, wine list, service, and atmosphere are spectacular.
Hope that this info is helpful!
Catherine Wilson