Never, party till you know how to magic burst, and stun bomb tosses, and avoid pulling hate from a NIN tank, and how to make yourself an attribute to a party (backup healing, sleeping links, enfeebling mobs) instead of the stereotypical melee viewpoint on black mages ("overnuking, hate pulling noobs who take 3 minutes after a pull to be able to get up and do it again.").
Only then, and then only, I might allow you to solo a little. Build up some soloing skills. Learn how to pull with Gravity yet avoiding links. Learn how to conserve your MP effectively, learn how to time your sleeps to a mobs attack so you don't get interrupted and die. Learn how to effectively use stun to run past a mob if you're cornered.
Then, when you feel you've learnt those skills, and you still feel an urge to go piss your life away with black mages who don't want to learn those things and would rather get 'faster' exp (which is BS, go duo or trio on wamouras with another BLM and a brd or rdm, great exp.) then sure, you can go manaburn.
I go manaburning maybe once a month, so I realise how lucky I am that I can solo fairly well.