Your alignment would have to be pretty damn out of spec to cause a shimmy in the steering wheel and chances are if it was that bad you would have noticed it before having the new wheels tossed on (although the larger contact patch would exagerate matters). Id put my money on a bad wheel balance job. Youd be surprised at how many places cant correctly balance a wheel. And if that turns out not to be the case you might have a wheel or two that is out-of-round. Any good shop that can balance a wheel should tell you if the wheel is out-of-round since it is pretty obvious when you toss them on the machine