SMN should never 'offheal', the dps you're not doing because you were spending it casting physick would be more important- you get the fight done faster. The only reason ACN, and therefore SMN, has physick is so that if SCH syncs down below 30 for content it still has a form of healing. It's scaled off MND and is worthless for SMN.

RDM I dont know exactly how vercure scales (I've heard it scales off MND, but has traits to help), and in videos you can see it doing 5-6k cures. That's better than physick on SMN, but a dedicated healer should never need to rely on a dps to help cure. It is mainly for an emergency, and most likely to keep with the theme of RDM having access to black and white magic. If you ever end up in a group as any DPS where the healers are mad at you for not helping to heal, that is entirely their own fault.

Now, if healers are dead, or low on mp, or most of the party is dead, you absolutely should help them raise people. Thats as far as your support should go as SMN