As a cabinet-maker, I have some knowledge in wood characteristics. You can only control expansion/contraction to an extent. That said, properly sealed wood won't affect playability regardless of seasonal changes. You also must consider moisture content, among other things. Think of burl (not Ives)........look at the numerous grain directions, but it's vastly used. Mahogany vs. Ash Vs. Maple and on, and on, and on. Different grian patterns, sizes, etc. Different movement, and so on. Grain movement discussion can go on forever.
String tension acting across the grain vs. with the grain shouldn't be a huge deal, either. How much pressure is being applied to the fulcrom of the bridge screws, strings, nut? Who can answer this: what is the shear-strength of your bridge screws? Ask a Structural Engineer if you need more in-depth definition of anchors relating to strength/tension/strength.
Sound vibrations travelling 5X's faster in one direction than another? I'm not literate on that topic, but I'd be interested for the statement to sight references so I can investigate further.
Aesthetics would be the main reason to consider running the grain parallel wiht the neck. Mandatory, not by any stretch.