I use Gmail and I think Google is doing a very nice job with their SPAM filter. However, I still occasionally find an important email that's stuck in my SPAM folder. That's why I scan through my SPAM folder every day. I also happen to have a Yahoo account and I probably find more important e-mails in the Yahoo SPAM folder compared to the Gmail SPAM folder.
When's the last time you glanced through your SPAM or JUNK mail folder?