I'm thinking about putting flat or riser bars on my commuter bike. I've got drops on there now so i'd have a .6 mm difference between bar and stem size. Would it be asking for trouble to use fashion a shim of some sort over the bars or would I be relatively fine? Have any of you tried this?

It's done a fair bit. You can buy commercial shims that are guaranteeed to be just right or you can make your own with a beer can, but it may take a few tries to get it just right. You want the shimmed bar to be pretty snug in the clamp already before you start tightening the bolt. Otherwise when you torque the bolt down you may stress the aluminum clamp excessively and risk a disaster.

I just shimmed a bar in place last night. A little strip from a Tetley's British Ale can, and away you go.

Granted, I have a Salsa steel stem, so I did a nice big shim and just pried the stem open a bit. No need to worry with steel stems. However, I have also fit 26mm bars in 25.4 alloy stems with a bit of brute force and never had a problem with the aluminum ...

good to hear...I'm poor and don't want to buy a new stem.

I bought the nitto shim because I had the money and I was buying a ton of stuff at the time of that purchase. Beer can, or another metal with a close enough thickness.

I have done it to make a Cinelli stem fit a Profile Bullhorn type of bar. Drink beer, cut up the can, shim, tighten bolt, ride and be happy. No problems whatsoever!

z-flashing

A good hardware store should have sheets of brass shim available.

If you got a pop-top stem, I wouldn't even bother.