I haven't experienced that as a problem. I've had very, very few develop cracks when heating to make corrections. I use no oil or crisco or anything.
I try to do the major corrections while they're green, with steam. When they're dry, I use the heat gun. But you guys are really making me wonder what I do differently than you... maybe I heat slower, or dont get them as hot? maybe I've just been lucky :^)
I live in western Pa, one of the cloudiest and rainiest parts of the country. All of my wood is in my garage or shop. I do keep staves I'm working on in my drying box. In my shop now I have a dehumidifier running, so things are fairly dry and stable there in spite of what's going on outside.
I don't ALWAYS re-seal the stave's back after exposing a new ring. If it feels dry, 'rings out' sufficiently while bouncing
an end off the cement floor, or reads dry with my moisture meter, I don't shellac them and they're just fine... again though, maybe it's the relatively stable environment. Not sure why, but cracks while heat correcting, reflexing, or recurving hasnt been a problem for me.