If you ever use any open wifi, then its really OK to use every wifi. The difference in security is basically nil. If you are really worried about it, you should never use any open wifi, and WEP or WPA wifi (crackable in minutes), and really only use WPA2 wifi with a certificate (a big pain in the butt).

A much better way to be secure is to make sure you are using SSL/TLS/HTTPS for things that you are worried about. Those are very very difficult to crack. We are trying to make this kind of security information easier to understand here:

http://shareyourwifi.org