You CAN insulate the floors. I would do that if the following is true:
-The crawl space is vented
-There are no pipes in the area that will not be insulated
-There are no heating ducts that aren't, or cant be insulated.
The thing with insulating the walls of the space is you are then heating/cooling the crawl space essentially. It becomes part of your conditioned space.
If you insulate the floors and have pipes outside the conditioned space they are more likely to freeze.
If I had pipes to deal with I would try and locate them up tight to the underside of the floor so I could insulate the floor. Supply that's easy. Drains are trickier.
If I had ducts I would insulate them, regardless.
here is a acticle on the subject (from a mfg)