Minnie doesn't cough them off very often either but I learned something interesting from my vet about this. She was constipated a few weeks ago and he said that one of the reasons for it could be hairballs... I had no idea. He suggested this medication (I forgot the name of it) which you give them once in a while, just on their lips so they lick it and it helps to clear the hairballs..
Anyway, I guess its not a bad idea, especially if they're getting constipated which could cause major problems..