For quite a while now, I've been leveling my Sith Sorceror as Corruption, which is their healing tree. Getting Khem Val early in the game (Khem Val is a tanking companion) definitely helps this cause. I can't speak for other healing classes, but having Khem Val tank groups of enemies for me while I bubble and heal is excruciatingly easy. It's even to the point where I can solo many of the Heroic 2+ quests in my leveling zones, and I can easily 2-man Heroic 4+ quests with a DPS or Tank party member.
Things "do" die noticeably slower than if I were leveling as Lightning, or even Madness. I'll admit that much. I have swapped to Lightning today, simply because there is an abundance of healers on my server, and it's actually easier to find Flashpoint groups as a DPS than as a healing spec.
But to answer your question simply: yes, it is very viable.