I'm with Toned here... BGs seem to have 4 as a nice sweet-spot. In the larger BGs, I still prefer to have a healer friend run with me over having a 5th DPS. Arenas is also better with 4+Healer partner.

5-boxing was only really optimal for me for DK+4xRets, and only pre-Resilience nerf when you could kill people in 1 GCD in arenas. It was also nice for soloing heroics, but my primary motivation for soloing heroics has always been gear, not fun or glory.

Come to think of it, I've probably had just as much fun running heroics as a 4x team as I had doing it all by myself. There's something nice about playing with a partner. I am curious if the ole 4xElemental approach will work in Cata, where you rely on 4xHeals from an elemental to top off the tank from time to time.

5 just seems anti-social. With 4, you can always level up or group up with a buddy and have a lot of fun.

That said, I'll be dragging a druid along with me for leveling from 80-85. Almost as easy to level them up this way and might as well get an alt out of the effort.