Thanks everyone.
I’m sure steaming would work but I don’t think it’s usually necessary.
So far I’ve put Z handles on 6 bows, half with natural grain and half with straight grain. I haven’t really noticed a difference to be honest, but the natural wiggles are probably sturdier in a torture test. As long as tip alignment and tillering are ok and there is a good strip of unviolated back through the handle, things have worked out for me—same as a normal bow.
Here’s the sister stave to this bow, showing the rough out shape- two check marks
and a few other z handles— a maple board bow, a shagbark hickory, and witch hazel with birch bark.
I like witch hazel with heat treating. Makes nice sapling bows.