shimmy is not well understood. There are a couple of good articles on wikipedia on it, here is a link to one It is the same phenomena found with grocery cart front wheels when you go too fast if that tells you anything.

I think it's clear that certain kinds of misalignment cause shimmy to be more or less likely to start. It wouldn't surprise me if an out of true front wheel would promote shimmy. However, the main phenomenon is determined by bike geometry and tire characteristics