Remembering back on my aerodynamics classes- there is a burble of turbulent air behind the trailing edges. I'm not so sure how much some slight variations in balsa would effect anything measurable when you think of different airflows at any given time across the span. Aoa, yaws, and wing shapes would be major factors causing relative chaos past the trailing edge. All end up becoming vortices.

The Cessna 150 is thick with those dimples, or whatever you call them in the ailerons. The F-104 TE is sharp enough to slice skin.

I think my thinnest TE is about 1/8. That is thin enough for me due to hangar rash!

I tend to disagree with it being trial and error if designed and tested properly. Some people may approach the design that way, but there would be properties involved for the aerodynamist.

Where is George Hicks when we need him???