Last year my 6yr old got sick while we were at WDW and she ended up being really tired during the day so we rented a stroller 2 dys in a row. It was very inconvenient when we had to get to the bus and from the bus to our room without it. The 3rd dy we decided to buy one for $40.00 at Epcot. It was the best purchase we made while we were there. She fell asleep on the ferry ride from MK, it helped us carry the "extras" we had with us every day and also allowed the other kids to sit down for awhile when they got tired, not to mention it carried all of our purchases. Eventhough our kids are too old for a stroller, WDW is a different story, and if someone needs it this year, we'll definitely have it with us!
You may just want to purchase a cheap one before you go or purchase one off property when you get there.