You can dig all the holes ahead of time if your measurements are good :~D
But if you want to do that, I'd go at least buy the wood posts, and dig and set each one as you go.
I guess it really wouldn't give me a head start, though. I won't really have any daylight until I'm off work, either on the weekend or because I took time...and if I've got time like that, I might as well go get all the stuff.