I like to rough the staple board and let it set in the shop for about a year, you will know if it's going to have any movement in it by then. I too relieve the bottom with a forstner bit.
Hollow or solid fret boards
I prefer quartersawn wood, with the grain running at close to right angles to the FB.
I also hollow using a Forstner bit, but with holes leaving 1/4" or less on either side, and nearly touching down the line, and I seldom get any warp. I've had solid boards warp in almost every direction possible. I think a flahysawn fingerboard glued up before drilling holes helps with stability.
Bill, this is not quite a response to your question. My fretboards have an intermediate step between hollow and sold. I use a forstner bit and drill 3/4" holes every inch. Occasionally, the fretboard will warp after drilling the holes. So far, nothing I can't fix with careful sanding.
I prefer that the grain runs horizontal to the soundboard. Again, I have not done any testing of alternate grain patterns. Now that you mention it. I will try running the grain the other way on my next build.
Has anyone made a comparison of hollowed out fretboards compared to solid ones as far as warping is concerned? ? I know solids are quieter and it seems the hollow fretboard would be less prone to warpage than the solid one. Also the wood grain , and flat sawed or quarter sawn has a lot of influence on warping.