Personally, I go ahead and book one as soon as I know I'm making the trip. It's one less thing to worry about as it gets closer to actual vacation time.
The first time I had to rent an ECV, I called Walker from within the Parks, and they met us and brought out an ECV within the hour. Having said that, it was during off season; back when the Parks were (by today's standard's) nearly empty.
I certainly don't recommend that; during high season, it is possible that most of the reliable vendors (as well as the Parks themselves) will not have an ECV to spare, so make your reservation as soon as you have your dates locked down.
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