As a former police officer, it is never acceptable to shoot, taze, oc, or asp anyone who is wearing cuffs. If your have ever been in real cuffs behind the back, you must submit to the officer.
We had an officer at my department taze a teenager who was wearing handcuffs. He fell right on his face and broke his nose. The problem was just for curfew violation. Really stupid I think. I guess that is why I quit being an officer.