One of the frustrating things about this hobby is that there is an element of art that can obscure the science. Caster "A" always heat treats his molds and gets good boolits "from the first cast." Caster "B" just fires up the pot and starts casting and in a few casts is getting good boolits. Both assert that their way is the best, and in a weird sort of way, they're right. Both have developed techniques that work for them, with their equipment, alloy, weather, relative humidity, etc.
My Texas mom used to say, there's more ways to kill a cat than choking it to death on butter. The fact that some one else reports a different experience than mine doesn't mean they are lying. Just for them, something worked that I didn't find effective--or that I didn't give a thorough chance.
For example, I make it a practice to always clean a new mold. I boil it for 15 minutes in condensate from my dehumidifier with a few drops of Dawn. Other people use break cleaner and a tooth brush. Still others use other combinations for mold cleaning. In all instances you are removing trace cutting oil to prevent it boiling out of the mold when you are casting, preventing unsightly creases and rounded edges. Am I right, or are they, or are we all like the blind men and the elephant, each correct as far as our incomplete experience tells us. Yet none of us has the full picture.
To bring these meanderings back to point, I heat treat brass and aluminum molds after cleaning. I have found that, for me, it makes for good boolits faster. In the case of Iron molds, I haven't found it to make a dime's worth of difference. YMMV: in fact, it probably will.