I would ask a couple questions. First, is it a manual wheelchair? Second, will you be spending more time in one park (for example, MK) than the others?
For a manual wheelchair, if you are spending a lot of time at MK I would stay at CR or BLT if possible, with Poly as my second choice. You can walk from CR or BLT on level walkways, and you can take the ferry boat back to the TTC and walk to Poly easily. But to get to Epcot from any of these, you do have a pretty steep ramp when you change monorails at TTC.
If you are spending a lot of time at Epcot, I would choose BC/BCV or YC. I wouldn't choose BW, not because I dislike BW, but because you have a fairly steep hill to push the chair up if you choose to walk to Epcot. There is a level walkway that goes under the hill from YC/BC that was put there specifically for wheelchairs.
I agree that is makes sense to stay close to the park you plan to spend the most time in.
The walkway from BW to Epcot involves a steep path is because it includes a bridge that goes over the water. The bridge has to be high enough for the boat from Epcot to get under, which means a high hill to go up and down.
YC/BC are on the same side of the water as the entrance to Epcot, so you can walk there without crossing the water. That is the reason that the walkway from YC/BC to Epcot is mostly level (there is a hill, but nowhere near as big as the one to cross the water).
If you walk from YC/BC to the Studio, you will need to cross the water, so will need to go up and down a hill.
But, you can take the ferry boat to both Epcot and the Studio from YC/BC and from BW.
The one problem with the Boardwalk area resorts is getting to Downtown Disney, MK and AK. All of the Boardwalk area resorts share buses. On a trip in October, we walked from Epcot twice and used the buses to get to Downtown Disney. Both times the bus was standing room only when we got on and both times guests with wheelchairs were waiting and were told they would need to wait for the next bus.
A previous poster did not like OKW and wrote the buses were the reason why.
We have been OKW
DVC members since 1993 and travel with a DD who uses a wheelchair. We have had no problems with OKW buses. In fact, they are some of the least crowded buses. Since the parking is very close to each building and many people drive, there are less riding the buses. OKW also does not share with any other resort.
One nice thing about OKW is the boats that go between OKW and Downtown Disney. They are wheelchair/
ECV accessible and the trip takes about 20 minutes.
When you check in, there is golf cart like vehicle to take guests to their rooms. It is not wheelchair accessible, but if she is able to get out of the wheelchair, fold it and sit on the cart seat, she would be able to use the cart. We dont because our DD does not have balance to sit in a moving cart and her wheelchair doesnt fold.
If she is tired and needs a place to lie down and rest in the parks, each park has a First Aid station where she could do that.
You can follow the link in my signature to get to the disABILITIES Board or the disABILITIES FAQs thread.