I try to use edge grain for structural performance in the core lams, and flat grain for the lams visible under clear glass... because flat grain is much better looking, imo... but never measured them for strength differences.
Veneers? I think any difference in strength or performance would be minute and irrelevant. Regular thickness lams like those used in a glass bow? Maybe it makes a small difference, but not much. More depends on the overall quality/integrity of the piece. The best looking flat sawn pieces are often the most 'compromised', and best used as veneers imo. As a 'lam' in a thicker-cored backed bow, or a trilam? It(flat, quarter, rift) might be measureable, or there may be other benefits related to construction, set, durability, etc of one over the other, but again, much depends on the specific characteristics of the piece. I judge them each on their own merits as best I can. Which is another reason I like to cut, season, select, and grind my own wood for my own bows when possible.
I just came home with some massive chunks of black walnut a few minutes ago, for my workbench and gunstocks, and whatever, all quarter sawn... but now you got me thinking about some of the nice flat sawn stuff they had, and that I should have grabbed some for glass bow lams.
~sigh~ if I keep this up, I'm going to have to build a new building just to keep wood in.