I think that if you wanted a very strong PvE and PvP team, your best bet would be to run 4 Shaman and a Paladin. Shaman aren't tough like a tank, but as they get up in levels they can wear mail which makes them a little bit more effective at tanking heavy blows. The Paladin you should setup as either Protection or Holy (depending on your flavor), the Pally will be able to take the hits from even the hardest npcs (granted, you'll need to watch the Paladin's health if you go up aganist something like Talon King Isis), but doing Heroics shouldn't be much of a problem. Some encounters just aren't made for boxers though, and you'll learn as you go along which ones are more feasible than others. As far as PvP goes, the 4 Shammi + 1 Pally team really shines. If you're up aganist another multi boxer who doesn't have complete control over what they're doing - Chain Lightning them; you're guaranteed to take at least one of them down (no one has over 15k hps - which is what the initial shock should do @ around level 70 with 4 chain lightnings - 1 target, some moderate gear) and then if you're lucky, you'll also kill either 1 or 2 of the other boxer's teammates. Thus giving you an advantage.

Of course, this isn't an unbeatable combination or anything; but it's the setup that I'm running right now after weighing out all of the advantages and disadvantages of the common 5-box setups seen here (e.g. the 4-lock + 1 priest combo). And, I don't want to chop my arm off by saying this, but if you're looking at PvE solely, then you definitely are going to need some kind of tanking. If you've seen how any end-game bosses hit, you'll know quickly that: a) you need to survive for at least 1-2 minutes in order to defeat one of the bosses and b) Felguards don't work that well in a "real" tanking situation. The shaman + paladin setup will give you that longevity - plus with Windfury you're paladin DPS will reach a respectable level.

My 2 cents.