I have way too many cars, so some of them only get driven about once a month. In my opinion (see the disclaimer in my signature) if you take the car out for at least a 30 minute drive monthly, it should be okay. If it sits longer than that, put a charger on.
Note: Other than my experience, my opinion is based on several JFP Posts such as this:
http://986forum.com/forums/191794-post3.html