In my experience (and reading), the sapwood is good in tension, and the heartwood is good in compression, not unlike yew. So an all sapwood bow tends to take set, and an all heartwood bow tends to explode. In fact, ERC bows in general want to explode. And when they go, look out! I've made a few ERC selfbows that have survived, but if you want to make a hunting-weight bow, it's safest to back it.
If the heart/sap transition doesn't meander too much in your stave, you should try to leave a thin layer of sapwood for the backing. Follow the grain, especially if you don't back it. This is tricky with ERC, just be gentle and use good lighting, and you can follow the grain.
It's very difficult to introduce any significant bending from steam or heat, so don't plan on that.
I love ERC, in spite of (or maybe because of) its challenges. It grows like weeds here, it's easy to work, looks georgeous, light in the hand, and makes a very zippy bow. Good luck!
