I'm looking at one to fit a 34.9 FD (came with a whole used group) to a 31.8 seat tube and I'm curious how many people use shims. I know FDs are cheap and I was thinking of selling this one and buying a new one; but using a shim lets me just keep this FD for when I change frames in the future to something with a wider ST diameter.

Shims work fine. Shimano ones do I should say. Not the perfect setup but nothing wrong with them.