its personal preference.

controlling a 5th character in 5v5 is very hard and its far more effective to have a 5th healer as a different player. ( for pvp )

now for pve, its a toss up imo. Some instances are much more effective to have 5 ele shams or 1 resto...... again... there is no "perfect " way to multibox. Whatever your goal may be, there are effective ways of doing it, but people are finding new combos and setups daily. I heard there is a 4 box hunter team that is doing well on the US Vengence BG group.