You want something that won't rust, compress, or rot to use as a shim.
The best way to do this is first set the toilet down without wax. Shim it so it won't rock at all. Lift it back up, and reinstall with a new wax ring. If it isn't a big gap, I've often found that pennies, dimes, or nickels work, and you don't need a trip to the store. A shim is more universal since it is tapered, there's no trial and error.