Ash almost wants to split, just needs a bit of encouragement. The bark also comes off pretty easy, too. Might as well do that right away, too.
I've been ambitious enough to take an ash bow all the way down to a roughed out stage on the day it was felled, then cured it in a hot car for a week before starting on the tillering. Since the wood was taken down so close to finished dimensions it was unable to really generate enough tension on the fibers to split or check. But if you don't take it down to bow dimensions right away, you really should coat the backs with wood glue, polyurathane, shellack, or even cheap latex paint. All those finishes can be lightly scraped off later and will prevent the backs from checking.