14 Conjurer here. As of right now, my build is to more or less divide all stats equally between Int/Mnd/Pie, with a few here and there into vitality so I don't get 2 shotted in party difficulty leves. In most party situations, yes you will be delegated to the healer role, but this doesn't sound like its a problem for you because you actually want to heal (which is nice since there's no "dedicated" healer class like in FF11, and we need more healers).
Reason you want more in intelligence is because 1. It'll give your spells a nice boost in dmg (which you'll need more so when you're soloing), and most of all 2. It'll give you a bigger MP pool. This is especially important when you're not doing leves, because just grinding random mobs in a area and no means to refill your MP other than your 10 min ability strains your MP pool. But, said 10 minute ability always refills 50% of your maximum MP, so more intelligence = more maximum mp = that ability refills more = you'll run out of mp less/not at all. I was in a 3 man party the other day being the only healer and we were killing mobs for 2.5 hours solid, and never once ran out of MP.
Mnd is also important, but not as much so. Like in FF11, cures seem to have a "soft cap", meaning your Cure 1 will heal about 200 hp when you're main job is Conjurer, but slowly goes past that cap as you get more mnd (I've seen mine do maybe 208 one time). So is it worth it to pour more points into MND than the other stats? Probably not. I'd keep putting some points into it, if only to reach the soft cap sooner for spells like say Cure II when you get it.
Piety is the last stat, but once again takes a backseat to intelligence. Piety as I understand it is more or less like Dexterity, but for magic: Increases your hit percentage as well as your critical percentage. Now if you were doing nothing but healing, this wouldn't really matter much, since your heals will never miss on a friendly target. But once again, you're not going to be in a party 100% of the time, and when you're soloing you will be glad you put some points into this and aren't missing monsters all over the place.
Vitality is more or less a requirement, just because like I stated before you don't want to accidentally pull hate from your heals (and this will happen) and then die in 1 shot, because now your group has no healer and dies shortly afterwards. More hp = you survive longer = your party survives longer, easy as that.