Oh, mine do that a lot! When Shiva and/or Kharma have been naughty, they'll lower those tails, curling up the last five inches or so and wagging that little bit like mad. They all do the slow, wide wag that's level with their backs, sometimes hitting themselves in the side, when they're particularly contented. When they are friendly but wary of someone/something new, their tails are carried high over their backs and wag slowly, lowering and increasing in velocity as the new person/whatever is accepted.
I've always said if you'll listen to the tail of a dog you'll have a better idea of what might be getting ready to happen. The front of a dog will bluff; the rear end never lies.