I (and Dolby) say the surrounds (sides) should go directly to the left and right of the seating position. That's fine, if you're always going to be using a 7.1 mode (like Dolby Prologic IIx, or DTS NEO:6). If you'll be playing 5.1 content without the rear surround steering, I'd say place the surrounds slightly behind the listing position. But if you don't mind the expansion provided by Dolby or DTS, then having the surrounds lined up with your ears really does work best.
The rear surrounds should be placed just beyond the edges of your listening area. But it's more about angles between speakers than distances.
Dolby has a pretty good, interactive guide on their site:
http://www.dolby.com/consumer/setup/speaker-setup-guide/EDIT: Heh, John was typing while I was finding Dolby's setup guide. But yeah, we agree.
