The arm, that part that has the pins, does not more or shift. The pillar, the long relatively straight piece that runs down to the bottom of the soundboard, does develop a slight sideways bend over time. The soundboard on the other hand develops a belly and it's only when that happens that the sound actually stats to have a richer, fuller sound.
I use Spruce for the soundboards, actually started cutting my own Red Spruce last year from out property, but most commercial Harp builders use aircraft Birch plywood. We find that the plywood soundboards, though quite strong, dampen the sound