I agree that the rears should be balanced properly. It is amazing how many shops these days can't balance tires worth a darn. It costs people a fortune in tires and suspension wear. I also agree with the spare rim demonstration idea. You may have to get them balanced some other place.
If there is a "twitching" in the front, it just goes along with the fact that they do not know what they are doing. When I used to balance tires, I could make tires with rims that were not very true, spin up nicely.
