Elm is darned good bowwood, I've made a whole bunch from elm boards. If your grain is good no backing necessary.....if your in doubt then back it. I have backed elm with bamboo, and a lot of other materials as well....the best combo I've built to date is a white ash backed elm.
My boards are edge grained (fully quartersawn) though so it may be different on plain sawed lumber. Elm has an intertangled grain sort of like hickory, which will help you out with any tillering mistakes. It's not indestructable but it is good tough bowwood.
Just one old farts opinion
rich