I don't know if you're getting a liner or gunite pool but either way I vote to build it whenever you can and don't put it off. Only because if you can do it before the winter and decide to push it to spring "just because" you never know what the weather will bring. There could be crazy rain that delays the builder by a month. I'm in southern NY (right next to NJ) and everyone backfills with the dig dirt. It settles big time and if you let it sit over a winter and then do decking / landscaping you'll know it's been compacted. I wold put off something (like landscaping) till the next year in order to get a heater. You'll love it. You'll be able to swim 5 months out of the year. 7 months if you want to swim in April and October, though the air is cold then and I don't feel the need to. I keep the pool open for looks though If you don't get one from the start, dig and plumb for it for the future. You'll spend a lot of money (and make a total mess) trenching for it down the road.
^^This, yes. We wired and plumbed for one, which was an extra $400. We are going to try one season without and see how it goes.