I only solo box when I have to gear a new tank or main toon that cannot be geared with pvp items. That also helps getting the LFG achievement on that tank toon.

Once your tank is geared properly you need no other people at all unless you are going to raid ICC, ToC or Ulduar.

For all the other raid instances you can just get some randoms to help or try 5 box them if you want once you have all your team in T9 gear.

On my druids team I still don't know if I will gear the tank with T9 gear or pvp gear all the team together. Being druid tanks uncritable via talents, it looks like a stack stamina stuff for the tank.