Just thought I'd mention that tail rubbing is not always caused by worms or any other parasite, sometimes, as in the case of my foal it is a habit.
Glory is always up for a scratch be it from me or the fence post. When she was smaller she used to go underneath the chain across her yard and use it to scratch her back and the top of her neck. But the best spot to scratch of course is her tail. With a rug on she wont do it, but as soon as it's off she has a massive scratch-a-thon on anything that will stand still, and that includes her mother!