This is a good point Hearpe, and indeed it is most noticeable where the sides are thin – say about 1/4″ as most well made harps are. One further observation: sound waves travel best along the grain lines, so for optimal transmission the sides should be quarter-sawn.
That could be a problem though since the force from the strings will pull the sides in and possibly rupture without bracing. I know of one very well respected harper who actually removes those braces and reports no ill effects.
I’d guess that most makers are conservative, tending to over build – as Dave Kolacny says “I don’t think it is possible to over build a harp neck.”
For such a seemingly simple instrument it is really fascinating. For my part I would say “I don’t think it is possible to build the perfect harp!”
One can find these physical properties and much more in the US Forestry Service’s document (pdf) published by the Forest Products Laboratory. Search “physical properties of wood pdf”
Blessings all,
Biagio