Actually, some students in the lab I work in measure bike vibration modes and showed that they are well above the vibration frequencies you see in shimmy. I think I may have actually mentioned that earlier in this very thread. You can have a 5'3" rider on a 49 cm frame, and if shimmy starts there is an optical illusion of the tubes bending. They aren't. I used to ride a 55cm frame that would shimmy at the drop of a hat, and it looked like the front of the top tube was bending =/- an inch. If it really was, I would never have ridden that frame.

That being said, if I was tall I wouldn't mess with standard diameter tubing.