Summary: Insulate your crawl space whether it is a sealed crawl space or vented crawl space. You will save energy costs and increase your home's comfort.
How to insulate a crawl space is a complex problem with complex answers.
A sealed crawl space has one set of problems, a vented crawl space another set of problems.
Insulating either is not expensive but it does requires a lot of difficult labor due to trying to work in a confined area.
And no, insulating a crawl space cannot be done before the floor above is framed. In fact, it can't be done until the house is framed, weather tight, and the crawl space dried out.
You and your builder should read the article below from the U.S. Department of Energy - Energy Efficiency and Renewable Energy.
It is an intensive article and unfortunately, non-conclusive.
Aside from the insulation quandary, building a new home with a crawl space foundation, creates problems unique to crawl space construction, sealed crawl spaces vs. vented crawl spaces being one of the major ones.
In my opinion, if you can't build your house on a full basement, building it on a concrete slab, or a raised concrete slab is the way to go.
But, if for logistical or economic reasons you end up with a crawl space on a new home or an older home, read Crawl Space Insulation from the U.S. Department of Energy.
If you properly insulate your crawl space—in addition to air sealing and controlling moisture, you will save on energy costs and increase your home's comfort.
How to insulate a crawl space depends on whether it is a ventilated crawl space or an unventilated crawl space. Traditionally, crawl spaces have been vented to prevent problems with moisture...continued.
Source: U.S. Department of Energy - Energy Efficiency and Renewable Energy
Be sure to read my article about adding insulation.