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.

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.