Repeating what others have said, and suggesting that this be your general rule in any event. Networks should be considered hostile. Secure your individual connections.

Note that with WiFi you've got an additional concern over simple snooping. If you want to restrict unauthorized access to the service, you should consider options for doing this. Using authenticating proxies for external access, and firewalling off any protocols you don't explicitly want to support, should turn most of the trick.