Iride01 

I'm having trouble with your statement of using it for your brake levers. Other than the pad that some levers have built into them on one side, I've never use a rubber shim strip all the way around the bar for the levers, whether just a brake lever or STI. Their built in clamp bands have been able to take up the slack on every size handlebar I've ever put them on.

I'd want the direct contact of the clamp to the bar so they aren't squishy and move when force applied to them. You likely will have to use more clamping force to hold them stable with a rubber shim under them and you might over torque and strip the clamp mechanism or break the clamp band itself. So if marring is what you are worried about, just use one layer of vinyl tape.

For other things on the bars, like out front mounts and lights a rubber shim is fine. But brake levers sometimes have a lot of force put on them.