Some woods you can seal the back, others can be left alone. You can find out by leaving the back of one unsealed and finding out. Me, I don't like to chance ruining a piece of wood.
For me, if the bark comes off, then I seal the back. I would suggest using a few coats of unwaxed shellac (superb water-vapor resistance, and you'll need to slow down that water-vapor exchange as the stave dries). Removal is easy: Shellac can be removed with denatured alcohol or scraping.
You can also use poly, but I've become sold on shellac.