As the original pally/4shammy boxer, I have to say: it's really up to what you want to do.

Do you want to have the extra complexities of managing multiple classes? Dual-boxing is by far the way to go if you're one of the types who likes to min/max their character and is getting bored with the solo game. I'm a 5-boxer, and I do have 2-man groups that I regularly enjoy. There's a level of control that you lose when playing more than 2 or 3 characters.

Do you want to experience group material on your own? Most group material is considered instancing, and that's capped at 5. While people HAVE had success running 4 dps in a 5-man instance, it's really much easier with a "holy trinity" style group with tank/healer/DPS. Excuse me, not easier but more by design. Sure, you can plow through some instances thanks to pure burst, but if you're this type of person you don't really want to exploit the heck out of snares and burst to get things done -- you want the experience of running all the standard grouping roles at once and the thrill once you've achieved those heroics. CC classes shine in heroics, since you have plenty of time to set up /focus'es and macros for CC pre-pull.

Do you want to raid? Is your guild multiboxer-friendly? If your guild is new to multiboxers, you might want to start with just two characters in a raid. Guilds unfamiliar with multiboxing will probably not have a problem wrapping their mind around you taking up two spots (usually 2x dps), but boy will their eyes pop out of their heads when you ask to take up 1/5th to half of the raid. If they're cool with what you're doing (and this works well for casual guilds who don't have the people to fill out 10 or 25-mans), you might be able to pull off 5 -- even so, raids are all about reaction time so I'd minimize the number of classes in order to maximize your dps. Raids (at least pre-wrath) are all about min/maxing.

Do you want to PvP in battlegrounds? AoE really works well here, as do the typical arena groups if your playstyle is more to hunker down and defend. If you're an offensive player or you like to totally disrupt the BG en masse, warlocks spamming SoC and mages AoEing are the best way to go -- usually 4 of one dps class plus a healer (priest is great for AoE heals). Shamans work well when defending nodes in AB/EotS, or defending tower caps in AV. They're not as offensive as mages/locks -- I play with Suvega often (4x mage + priest) and he usually heads in first, ripping up their defenders and I follow behind, bursting down healers and then hunkering down next to the flag while he goes and caps another one. 2-boxers (dps + healer) also work well here.

Do you want to PvP in arenas? 5 characters is hard to do in arenas, and 4x Shamans plus a third party healer (at least pre-Wrath) really shine in this area. Keep in mind that, if you do have 5 characters, you can always "drop" one and arena with 4 of them. Arenas are all about burst, cc, and anti-cc. Shamans shine in burst and anti-cc (grounding/tremor). CC as a multiboxer is hard to pull off, since it usually requires setting up /focus'es (which really doesn't work well in the fast pace of arena games).

So, it really depends on what you want to do with the whole thing. If you want the instance experience, go all out with 5 or expect to LFG for the remaining spots. If you want to arena, you really only need 4 since you need a space for your healer. If you want to raid, it's based on what your guild is willing to commit to. If you want to PvP in battlegrounds, there's alot of options. If you just want to experience controlling multiple characters, you're going to get a really closeup look if you 2-box, since you have way more brainpower for individual control.


Why did I choose pally+4 shammies? Because my "main" was a shammy, and by leveling 4 shammies and a pally I could both round out my group of 5 for PvP, add a tank for instance and raid encounters, and potentially take just 4 shaman to arena with. It provided me with the most flexibility (thanks to the ability of hybrids respeccing from dps <-> healing) with group composition while still giving me the option of going "full dps", "dps + heal", and "tank + dps + heal"