Steam or heat? Hmmm. Steam IS heat, isn't it? lol
HHB takes heat corrections well with either steam or dry heat.
I steamed, straightened, and added reflex to dozens of HHB staves a few months ago. They were recently cut and had a good bit of moisture in them yet. It went very, very well.
If they have overall twist or alignment issues, I steam and correct the whole thing at once, and then when they're dry and I begin the bow, make any smaller corrections with the heat gun, and that works very well on HHB too.