Always heat from the belly side. If you accidentally a or u the wood on the back, it'll loose tension strength and break there. On the belly side, it will raise the compression strength. A light shade change won't so much, but it it goes black, things break.
Hickory LOVES dry heat. It bends easy with it, and when you heat treat the belly, will make it much faster, less set and a more stabile hickory bow.
Kyle