I feel more comfortable with AST as healer, but I use SCH when I'm confident at healing for the day (whichever DR I plan to get into).
- if you're familiar with the instance, and knows when a big AoE will hit the party, pre-cast succor for less spot heals/indomitability as the latter eats up your aether (time it wisely!)
- don't forget to put fairy on obey, especially when switching from SMN (I personally have my ifrit on sic all the time)
- pray you don't have to use lustrate often, but don't forget to use it for emergencies or after you've rezzed someone (while it's OK to spend your aether)
- if paired with AST, be wary of their stance, let them know to change it if they're on Nocturnal unless you're not planning to use your shields then carry on