Not a fan at all, I'm afraid. Dogs have a tendency to run whichever way they choose and not necessarily have any sort of trail etiquette. For example, I had a dog run up the trail at me, at speed, as I was coming down one of the faster sections of Whites at Afan. I had to swerve to avoid the dog, and was very lucky to have only crashed up the hill, rather than falling down it.
No problem at all with dogs and bikes out on the open trails, but I think that the speed and nature of some trail centre riding means that it's really not the best place for dogs. Sorry.