My theory, based upon experience, is as follows: Shimmy starts in the wheels (usually the front). Tires, untrue rims, whatever, it starts there. Forks, tubesets, headsets only serve to amplify the medium-high speed shimmy problem that the wheel is out of balance. Low speed shimmy is almost always a problem with tires. 99.9% do not balance the wheels on our bikes, but we ALL balance them on our cars. Balance your wheels (very hard to do), and the shimmy will go away. Sometimes just shifting weight (different tires, or even a stylish spoke reflector) will do the trick. Fooling around with headsets, forks etc is always a goood idea to keep a bike in working order, and may reduce the noticability of shimmy, but it will not eliminate it.