I'd say that T1 is worth casting at the start. I did some manual parsing on a dummy in Coerthas (not huge sample sizes, but enough for some napkin math), and it looks like a T1 with no crits is approximately equal to an AF2 Fire1 (which suggests that their damage formulas are the same since they equal out to the same potency as well). The thing is, at 251 PIE, your first rotation can only handle 4 Fire1s since your initial F3 is full cost. You'll have more than enough MP to spare for a T1 though. Below are some MP totals for standard rotations.
251 PIE MP pool = 3728
5 Fire from a cold start:
F3[532] + F1w/AF3[638] (X5) + B3[79] + T1 [212] = 4013 (I think you would need something like 283 PIE for this much Mana on BLM, unless PIE scales differently as you get more)
4 Fire from cold start:
532 + 638 (X4) + 79 + 212 = 3375
4 Fire w/T1 from cold start:
212 + 638 (X4) + 79 + 212 = 3587
5 Fire Rotation starting from UI3 (just for reference):
133 + 638 (X5) + 79 + 212 = 3614
So yeah, there doesn't seem to be much reason NOT to start with at least a T1 at the beginning of a fight. You don't give up anything MP wise, and it's an efficient enough spell to be worth the cast time IMO. You could actually spare enough MP to bump it up to a T2 if you wanted, and then it would be 25 potency MORE than an AF3 F1. T2 is a longer cast though, so it will delay your going into Fire Phase by an extra half second. If you decide to open with Swiftcast, however, then I would recommend it.
Edit: Had T2 potency wrong. I forgot the extra tick, which puts it ABOVE an extra AF3 F1. Definitely worth IMO, since the duration is very likely to synch up with the length of your rotation anyway, and you probably won't waste a single tick. Probably worth even without Swiftcast.
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