Unless you want your entire neighbourhood to use (or abuse) your wireless network it's probably a wise idea to tighten up the security a bit. ExtremeTech published a guide on how to do this:
I've gone so far as to instruct neighbors, who admit they know nothing more about wireless networking than how to turn on the equipment, on how to lock down their networks. While I live in a relatively remote area where it's unlikely that wardrivers or leachers (people who drive around seeking unsecured networks from which to steal data) might pass through, there are still risks to leaving a network wide open.
If you own a wireless access point (AP) or router and a wireless-equipped computer, you can reasonably secure your network without spending a cent on extra hardware or software. Nearly every AP and router contains some sort of encryption, and you can take a variety of other steps to make your network a less savory target for pirates and hackers.