Is hitting a curb at 20 MPG reason for a wheel alignment?

Your call. Depends on you angle when you hit it, if you hit it square, then not really. If you hit at an angle, then I would recommend you get it aligned. The cost of the tire alignment will be relatively small compared to the cost of new tires if it does need an alignment. General rule: If you think you should get something fixed on your car and it won't put you into the poor house, go for it.