Personally I've found Psysick invaluable for doing the MSQ and job quests, to heal myself and NPCs. I was not capable of doing my LV 45 BLM quest without it, and since I hadn't bothered with cross-class skills then, I had none, except for archer skills (up to LV 15). After I switched to Arcanist to level it up to LV 30, went back to do LV 45 BLM quest and I could do it, heal myself and that stupid NPC. It also made doing MSQ easier by either healing myself or NPCs. One of the MSQ quests all I did was using Psysick on NPCs on my BLM.

After that I got my cross-class skills first before leveling the jobs I wanted to play.

Potions are really expensive (the highest level), you can only use 1 bottle per 45 seconds or 1 minute or 2 minutes so if you need HP NOW and you have to wait 1 minute and 30 seconds before you can use that bottle again, there is a good chance you will be dead then. Also, you cannot heal NPCs with your bottles.