Eric, primarily with whitewoods to help increase the compression strength. I heat treat most bows but not as much with osage, etc. that are compression strong. Marc St Louis, who wrote the chapter on heat treating in TBBIV, told me about adding the turpentine/pitch to the hot wood. He said it replaces some of the resins that are force out by the heat treating. I'm reducing the weight on this bow quite a bit so I've lost most of the char color and probably some of the resin too.
I use Tru-Oil for a finish and it protects the wood and slows the hygroscopic rate.