I run 4 shammy + 1 prot pally, did it at 68 without any deaths. If its 5 shamans, I would suggest making sure you have macros that lay down rotating stoneclaws, using rotating frost shocks to kite, using earth elementals on melee mobs, etc. The 70 instances are not really ones you can stand toe-to-toe with mobs as a shaman and tank it, you have to kite and kill stuff before it even reaches you.

If you want to do heroics in WOTLK, I would suggest perhaps leveling a tank, the heroics are somewhat easier than TBC but the mobs hit very hard and with the changes in resil (resil having no effect in instances) I think it will be hard for 5 shamans to do heroics in WOTLK, unless way overgeared.