Aloha,
I have read the theorycrafting threads in this forum and I am very aware of that everyone and his grandma is recommending a prot paladin and four shamans. I believe you, it's probably the easiest and most efficient group. It's probably just me, but I find the idea of playing four characters of the same class (and three with the same spec) a little dull.

First I ran a 3-man group with a prot paladin, a fire mage and an elemental shaman. Fairly standard and nothing new under the multiboxing sun. When they reached level 60 Blizzard launched their Recruit-A-Friend campaign, so I decided to buy two more accounts and level up a bunch of more characters to have a pool to choose from. My intention was first to use the casters for multiboxing and do some solo arena and PvP with the melee characters. But I soon found out that it wasn't harder to multibox melees than casters.

I have cleared all instances up to BRD with this group (they are around level 54 now):
Protection paladin
Feral druid
Combat rogue
Fire mage
Holy priest

And I cleared Hellfire Ramparts the other day with this level 62 group:
Protection paladin (not the same as in the previous group)
Fury warrior
Elemental shaman
Fire mage
Holy paladin

Positioning the melee characters in front of the mobs is pretty simple. I have a key that send a /follow macro to all "melee" WoW windows. After I have pulled the mobs and built up some initial aggro I just press the key and directly follows up with a /startattack macro. My melee alts will come running towards me and as soon as they are within attack range they will hit the mob and automatically stop. I might have to move a little to position the mob in front of them, but usually that's not needed. If the mob runs away or knocks my guys away I just need to repeat the procedure again.

A pure caster group (with a paladin tank) is probably still more efficient, but melee units have their benefits too: When my tank loses aggro to an evil mob it usually makes liver paste of my squishy casters in a couple of seconds. But when the next guy in line is a fury warrior in plate the whole group will stand up until I have returned the aggro.

I'm sure that there are more multiboxers who have tried to run instances with mixed melee and caster groups (or perhaps pure melee groups) but you just have been silent here on the forum. I would love to hear your experiences!