Depends on your speaker impedance and length of the run. You can have a look at a table if you like, but unless your run is over 20 feet, 16 gauge should be fine for anything.
Only other thing to consider is if you are adding any ends. Some ends are easier or harder to put on larger or smaller wire. For example the ends I like from Blue Jeans Cable fit really nicely on their large 10 gauge wire. They'd work on 16 gauge, just not fit as snug. Conversely, some other ends have real trouble with big wire bundles.