I don't really care that much about the 5man content, but i've been able to get through most TBC heroics with 5 shaman without any major issues. That said I do think having a pocket tank ready for wotlk is a good idea, especially if you have the gift levels for one. Big appeal to me is for naxx 10, atleast in my guild there are very few tanks so the 10man runs sort of revolv around what they want to do. If your bringing the healing and tanking to a 10m, you can fill in the rest pretty easily with dps and make the runs happen more around your schedule.

So im just leveling my 5 shaman 70-80, then after they are 80 ill level up my other chars as a second group. You get a free deathknight thats level 55 so the benefit of waiting to level until wotlk is that you can take the deathknight up with your other chars 60-80