Self-parking has been a problem at the BWI/BWV - especially in the late afternoon/early evening time frame. If the lot is full, it's full - you will get directed to the lot across the street, which is a long walk from the resorts. If the self-lot is full, you will definitely want to valet park (even if you have to pay). The "closer" self-parking lot generally clears out after IllumiNations. I wish the BW managment would not allow non BWI/BWV guests to park in the BW lots if they are going to EPCOT, but I suppose they can't figure out how to enforce it without negatively impacting the BW merchants/restaurants.
Best wishes-