As much as casting Fire III first will grant you max power for BLM in one spell, Using Fire I 3 times you will eventually get the same result, I normally don't care about little details like that. However, what can annoy me is if they don't switch from Ice to fire and Fire to Ice accordingly as if you exhaust all you MP and "refuse" to switch elements essentially you're a sitting duck, then I would be annoyed.

Play the Job however you want, BUT at least make sure you're doing the 1 thing that really matters to the job.