It is never a bad idea to seal up the back of the bow. Even if the stave is perfectly cured and would never check...sealing it isn't going to hurt a thing.
Where I live the relative humidity can be stupidly low. I have had osage staves that were cut for 6-8 years shipped here and they immediately checked when I chased a growth ring! So I chase a ring and seal the back even before I start to clean up the shavings from the floor. If I get partway through and have to stop to pee, I seal that sucker!
Your mileage may vary, depending on driving conditions, barometric pressure, and the rising cost of copper ore in Patagonia. A pint of cheap lacquer, polyurathane, or even 50-50 thinned wood glue is cheaper than a wrecked stave.