Pre 40 you can go shadow, you can go holy/smite, you can go shadow until mindflay and then go into disc. It doesn't really matter much tbh, as long as you have wand spec, spirit tap, and improved sw pain.
40 you certainly want to respec to shadowform. You can run with this until your late 50s, respec to a healing spec and get your gear. If you can holy nova farm, go do that. Otherwise you can go back to shadow for farming while being able to heal any 5/10 man now that you have a decent amount of +heal gear.
That's at least how I've done it in the past and plan on doing again.
