You'll want to use a material which has similar properties to the column or stronger. Nothing wrong with a metal plate, but if the columns are PT lumber, then a PT shim should be OK. Wood does compress more across the grain than along it, but not by enough to be a big deal in a deck. Cedar is also pretty rot-resistant. If you've got an inspector coming around, the metal plate others have suggested may be better, if for nothing else than because it looks like you took it seriously. But the way to actually take it seriously is to replace the column.