If they are not there yet, you should probably add them, because such an address book of everyone you know can be a helpful tool to identify spam.
Assume Mail from Unknown Senders is Spam
If you do not usually receive mail from strangers, you can assume that
- every message not from somebody in your address book is spam and
- filter such messages to the Junk Mail folder.
Now and then, you should check this folder for important messages you may have missed, maybe because somebody's email address has changed.
Building on this idea of only allowing known senders, challenge/response spam filters render your email virtually spam free with very little to no maintenance — but annoying senders of good mail.
(Updated February 2012)