I leveled a Sorcerer as healing until just recently, my advice is don't unless you seldom do anything other than group/PvP. It's painfully slow to quest with in respect to the other two damage trees, and quests are the quickest way to level. Your companion won't have dying problems if it has adequate gear for the zone you're in, and you can still toss heals. If you're still worried, carry around some medpacks that heal yourself and your companion in case.