and a good idea. You know, if you use fiberglass batting I would definately lay down a heavy plastic vapor barrier. Since you have the lux of space between the floor joists, which I hope that you go 16 inch OC, you can lay the insul between the joists. The compressed insulation boards are nice where the lux of space is not there.
Normally floor joists are 24 inch OC, but a floor that waffles is a poor floor and to eliminate that 16 OC is recommended and 3/4 inch ply for the sub floor is also rec.