Suzanne, as usual, I agree with Jay J, but not necessarily for the same reasons. Your floors were originally milled with a vee groove and trying to fill them will result in obvious "streaks" in the floor where the filler was used. You or a contractor will never be able to get the stain and finish to blend between the fill and the wood and this will look weird. But one other note is that with each sanding, the width of the actual grooves will appear smaller as the top layer is sanded off.
And no matter what you do, the floor will expand and contract and wood does this more across its width than along its length, so the chances of the fill popping out are greatly increased.
This does not mean that you should not use a filler on the floors. Where you have asymetrical spots to fill, the finish blending will be much better and look more like natural variation in the wood.