Most random open wireless networks you find are "open" are there to find because the owner doesn't know how or is too lazy to turn on and configure the security features that would prevent you from finding it in the first place. Odds are they won't even know (or in some places, won't care) that you're even there.
There are, as always, exceptions to this. Might be someone with too much time on their hands just waiting for someone like you to find their open wireless network to do all kinds of bad things. Hey, I know a couple that do (really sad, no tan at all from lack of going outside) that kinda thing for kicks. But honestly, you're probably safe.
Just in case though, follow the advice from jsw above and you should be ok. Unless of course, you have some top secret info that no one else should know and other people know you know it