depending on the size of the cracks, type of wood, aclimation at installation, etc, it's hard to tell what would be the right fix for your floor.
is this a new floor? if so was it allowed to acclimate properly (let sit in the house with hvac running at constant normal temp for AT LEAST 48-72 hours)
what type of wood, and width was used?
how big are the cracks, and on what side of the board (ends or edges)
what general climate or area do you live in?
if the cracks are large, any filler will work itself out eventually. if small, and you just can't stand it, you could fill, but I wouldn't recommend doing in alot of areas unless floor was being completely refinished and filler was UNDER your finish.