Well, technically, you don't NEED to do a wash on it. It's really a matter of personal taste. It can bring out some of the fine details, and give it an overall "used" appearance. If you want a fairly foolproof wash, do what I do and use black & brown tempera paint (yes, the kind they use in grade school). It thins and cleans up with water, and if you don't like the way it turns out, you can usually wash it off under the faucet and start over. Mix a 50/50 (50% paint, 50% water) solution in a small cup, and you're good to go.
Regards-
Mark