When folks come out for herding lessons with me, I tell them they are the shepherd. The dog must work and respond to them; first, last, and always. It's my feeling that if the dog has the ability, even if it is a cross, it should be worked. I have had folks who "asked" their dogs to work rather than projecting as a leader. Three stages of commands: ask/directive, command, demand.
When you ask, it is not an option, your voice should be positive. "Lie down." "Walk up." Step 2 is the command and your voice is stern. Step 3 is the demand and you step to the dog. If the dog lies down at that point you back off. I know of many "trainers" who go up and beat the life out of the dog for not being ROBOTIC! Anyone can beat the hell out of something and call yourself a trainer.
Real trainers look beyond the problem and try to see the cause of the issue. This mindset also works in the protection area od Schutzhund. I have worked with Rottweilers, German Shepherds, Border Collies, Shelties, BC crosses, and my Bouvier (Bear) and all have different comfort levels with sheep. Shelties are talkers and at 15# can get their butts into trouble with a 200# ewe.
I would start in a round pen with some well dogged sheep, Katahdins. Keep the dog on a long line until you are sure it is not livestock aggressive, then enjoy. It's going to be great in 2008!