will present a problem because you are staining a different cut of the wood. The surface will stain nicely, but inside the grooves, the stain will be darker due to the cut.
I would soak each dentil or the ceiling molding in a good sand sealer. this will make the different cuts more the same when staining. I would use a light stain. If this is pine, then it does soak up stain quick and fast.