I haven't multiboxed a 5dps team before, but I've done it just playing one toon - here's some thoughts.

5 dps is a "shock and awe" campaign. You want to hit them hard and fast, and hopefully take them off guard. In fact, when I play on one (or see one play against us), we (they) mount right up before the match and ride right over to us, again, vying for a surprise element. I won't say 5dps can't be effective without doing this, but you lose (at least what I consider to be) a big part of your advantage once they figure out that you're 5 dps because they can prepare for you. Obviously, mounting up and riding into the opponent isn't what a shaman multiboxer wants to do since you'll be without totems.

The other thing is, as you go up the ladder, you'll find better geared opponents. As this becomes true, you'll have more trouble insta-killing one target. They will also be a better coordinated team. All of this means you might not get that first guy down so easy, they will be on you, and without a healer you're going to be in a bad position.

That's my thoughts anyway. Good luck with the team.