Actually, this one has a little truth to it and is not entirely a myth. A dog scoots on his rear in an attempt to relieve irritation - and worms can be one source of that discomfort. They aren't the only potential problem, however. Impacted or infected anal sacs can be a problem, too, and so can a piece of something a dog has chewed that hasn't been completely expelled in the stool. For long haired dogs, fecal material can cause a nasty, itchy, and oh-so-smelly mess that needs to be cut out prior to bathing.
You need to keep an eye on this area, whether or not your pet is scooting. Express the anal sacs when you bathe your dog and keep the hair clipped short and clean around the anus. (Your groomer or veterinarian can take care of the job for you if you'd rather leave this task to someone else.) Be observant for signs of worms on your pet's rear, such as the squirming segments of a live tapeworm or the rice-like appearance of dried segments.
If you think your pet has intestinal worms - because you've seen them on his rump or in his stools - have your veterinarian confirm your guess and prescribe appropriate treatment. Be on alert, too, for swollen areas on either side of the base of your dog's tail, which indicates infected anal glands, a condition that also demands your veterinarian's attention.