Alissa: it wouldn't be awful giving her anything, but she's aiming to avoid combat completely for the next twenty levels - she can manage without.
Altha: Her HP isn't bad, but her def is in a similiar situation to Shae - it's not really worth improving, because it's literally rock bottom and her growth is pretty low. Maybe with some good rolls it'd become worth sinking resources into, but otherwise perhaps not. She's a mage, with range on all her weapons, and attacking from afar is what I assume she's going for with that low def.
Arnold: HP is pretty good, def is good, he'll gain large amounts of health regain, and as a pure healer he's staying out of combat. Pass.
Cecil: going to end up with tons of health, and def isn't awful? He can absorb the damage with the meat of his face. Wouldn't be atrocious him getting the def, but definitely not the hp item.
Flora is going to be both a combatant and a healer. It's important to keep healers up, and her def and Hp are both alright-ish. She's going for throwing ages, so will mainly be at range. I don't feel one of the boosters would go to amiss, and i'm not just saying that as her player
Freddy von Henny: Great health, pretty good def. He's fine.
Gregzeck: Like Fred but slightly worse.
Iris: Low health growth and def could be better. Like flora but with less hp, which isn't brilliant for a fighter. Could easily do with either of the upgrades.
Jann: See fred again
Shae: I'd say it's not actually worth giving shae the def because her defence will never amount to anything anyway. Especially with that cheeky PF. Her hp is in a similiar situation. She's glass cannon to the max, and I'm unsure it's worth it to try to compensate for that. Vantage is her defence.
Friedsieg: Plenty of health and def again.
Suzanne: a thief, so more useful running about and nicking stuff. That said, her health is pretty dismal and def isn't great, so perhaps it's be worth considering saving a treat for her.
My thoughts, at least.