That 's the correct way to use pour on dewormers. I think the official recomendation is every six months but we use it on cattle only in the fall. (I use diatomacious earth on all the small animals so that there is never a residue so that I can buy\sell\butcher as I please... Only the cows get chemicals.) Worms grow in your grass and require that the animals eat the eggs to continue the lifecycle. I deworm the cows in the fall to kill whatever they picked up over the summer. They don't really need it in the spring as the're not on grass through the winter.
If money's not an issue, do whatever it says on the lable just to be sure.