What I did till 40 was mix it up with Disc/shadow, then at 40 went full shadow, and then added disc, (wands and Imp Fort), then topped off with shadow. As a priest, everyone assumes you are a healer, and you can heal low level dungeons fine as a shadow priest. I still heal instances like Underbog and Mana Tombs, (I am now 64), and if I am in a PUG, they do not know I am a shadow priest unless I tell them. Do not be tempted to go shadowform if you are in a instance as the healer, though. All of your mana should be to heal, not damage. Feel free to wand a mob near death to help if you wish though.

For healing, about level 50 is where I find off spec pali's, druids, and shamans have a hard time healing. For priests, though, I believe off spec we can heal up until raids. Even in shadow, don't underestimate your healing ability. If I run shadow, the main healer never heals anyone but the tank. I keep everyone else healed. I have even been highest in heals once, over the healer, and still #2 on dps, (damn mages with their AOE). To be fair, we had a good tank and he didn't need a lot of healing.