My warrior in my mixed group never ever targets anyone except the mob to kill. If the healer gets a mob on it for whatever reason, I reach over and push fade. Or I run the tank up to the mob and do my attack macro that has sunder at the start. Or I send the hunter's pet at it. Or I just bubble the healer and wait it out. I have done everything up to DM and never had a problem. But hey, everyone has different stories.

With my shammies, they are built to take hits. I would want each of them to take equal number of hits. It makes healing totems and chain heals easy for me. But hey, for beginners, it should be really easy to make 5 different macros to heal 5 different people that won't work if you change your group line up.