True. Mostly. If you are REALLY paranoid, and you have the 802.11 enabled, someone could get on your 802.11 network and would then be behind the router firewall, allowing them to access your PCs if they had the further hacking ability to do that. It is relatively easy to get into an 802.11 network, even an encrypted one, without any hacking skill.
Of course, this person would have to be parked in your driveway for that to work...