CATXTransplant is right that you might end up paying for an inspection twice and not need it. However, I still recommend hiring an inspector before closing, as well. IMHO, there are a few important reasons for doing so:
1. If an inspector identifies any major problems with your house that would be a deal-breaker for you, it is a lot easier and cheaper to call off closing and walk away from your earnest money than to be stuck with a home you now own and are trying to have fixed.
2. If the inspector identifies items that are not deal-breakers but simply aren't done correctly (and you wait until next year to find them), you'll have to deal with being uprooted by construction work while you're living there, and you'll have to rely on your builder to schedule the work to get done in a timely fashion.
It's much easier to have things fixed before your furniture, family, and belongings are moved in, and you'd be surprised how much more quickly builders will resolve things when a closing date is on the line.
Finally, if the item were something like a leaky vent (or, for example, a HVAC return that was never installed and drywalled over -- an inspector just found this in a client's new home), you could end up paying for it in other ways (like higher energy bills) for the next 12 months.
Of course, most new homes should be fine and not have major issues to worry about. Most of my clients buying new construction are perfectly happy, but there have been a handful who were definitely glad they hired an inspector. In the end, it can be worth MUCH more than the $400 you'll spend to make sure everything's done correctly.
I worked for a big builder before becoming a Realtor, and I've seen many of the things that can go wrong or be missed. The big issues are rare, but it's better to be safe than wonder if any of them are hidden in your house...
That's my $.02.

Best of luck with whatever you decide.