It's always nice to ask, although silently checking your e-mail and a couple of webpages once may result in far less trouble than telling the generous neighbour that they are *****s (using unsecured wireless) and that you want to steal stuff from them (it is unbelievable how wrong some people understand other people).
Always use encryption on your wireless network, even if you don't mind sharing it with your neigbours (and please tell this to the people silly enough to have unsecured wifi around you). Unencrypted wifi means that anyone can come and listen on your internet sessions.
Many people do not realise that most internet traffic in unencrypted and an eavesdropper can see the contents of your e-mail sessions (including passwords and message content), instant messaging (depends on the service) and web pages (including passwords). This is OK if you have a wired connection, but with wireless it's a serious security risk. I'm not even going to mention that your wireless is inside the router firewall and you are wide open to any malware unless you have software firewalls on all your local computers.
Try to use WPA, it is far superior to WEP. Although cracking WEP is not difficult, it is still better than nothing. Cracking the key requires some effort and the evil wardriver may have to sit in front of your house for several hours (and risk detection). A casual wifi user will probably just forget about it when they see the "encrypted" label next to your SSID.