Last time my legs were measured (several years ago, 35 now) my legs had a difference of 18mm, mostly in femur length. Been cycling since high school and I tried shims when I was in my early 20's and working at a shop that was very proud of their fit process. Caused problems for me that didn't exist before, back and hip pain. Dumped them and haven't had problems on the bike since after several different road and MTBs over the years. To each his own I guess.

I'd second what a previous poster said about starting gradually and not shimming the full difference right away.