It's a completely different playing field with more than 5. It is more difficult to que for pvp though. I know that valk enjoys his 8 shaman, but i know there were plenty of times when only four made it in to our games.

I think you should play as many as you can afford, and would enjoy playing. I am not sure on your main focus, and if you play with friends, as that could possibly change things. If it is for solo play, then you essentially could play whatever you want..The problem with 8 boxing is that you might as well 10 box.

Leveling two sets of four is great, but doesn't matter anymore with the looting changes. The problem with 8 is that you really cant run 5 man dungeons, unless you play a tank or healer.

I personally think that leveling a team of four is more fun than a team of five.