I have never had a problem in EWR at Avistar - including when we left on the Friday of holiday weekends. BUT - I've never tried it around Thanksgiving or Christmas. I'd make a reservation just in case.
I made a reservation once, just in case, and when we dropped the car off, I told them we had one. The person said, that's ok - you really don't need it.
It probably is different at LGA, but I'm completely happy with Avistar in EWR.