I always tell them up front. When they ask if we're on the dining plan I always say no but I do have TIW. Never had bad service because of it.
TIW is not a dining plan. There is really nothing to gain from telling them ahead of time.
I always tell them at the end of the meal.
I always do, I feel like I have a better chance of actually remembering to use it if the server knows about it too, but I am notoriously bad with remembering to use things like loyalty programs, discounts, and coupons.
I have never felt like I received poor service due to the server knowing about the TIW ahead of time, but it isn't something that has ever really crossed my mind to think about.