I have a shepard/lab mix breed that I take with me on my coyote line. I'm still fairly green at coyote trapping and the dog has put me onto locations that I would have more than likely overlooked. This past season I can count 3 coyotes that I caught that I can directly attribute to the dog putting me on location. and one of those after he showed me where to put the trap, he trailed the animal when it went for a run with the drag.
I agree with Ric, that unless a dog is well behaved it has no place on a trapline.
If you're unsure about taking a dog along with when actually trapping. It can be good learning experiences taking him for walks in the off season.