Lately i like to do mine as i approach low brace, when the bulk of wood removal is done. But that's just me and my way
To verify what Chris said, i did a really deep heat temper on a hickory pyramid stave and it gained an insane amount of weight. I removed all the "toasted" wood and then some and it was still VERY tempered. I suspect the tempering was still evident after i'd removed half the thickness.