Ash makes a very decent backing and it's easy to find a suitable piece since it often has straight grain. Of course, it also depends on the wood you use for the belly of the bow. If you use osage or ipé, you would be better off by using a backing with an even higher tensile strength such as hickory of bamboo. But if you want to use cherry, walnut, maple or oak for a belly, the ash will really complement it nicely.
Don't forget to check out the white oak as well; it can be at least as good as hickory for a backing...according to Tim Baker. I've got some white oak backing strips myself as well (haven't used them yet) and they look really promising. Easier to get than hickory.