I've just started running into more and more one-time use buffs that permanently increase resistance to poison, fire, etc. My first thought was to give them to my thief but then I thought maybe it was best to spread them around as much as possible.

Thoughts on this?

I usually spread them around. Elemental damage is rarely done to 1 party member only (except when unlocking chests or doors, which is why I suppose you gave it to your thief). Still usually it's either a trap, a lava square or a creature attack, all of which does damage to all party members.