That was GREAT, Dan! Thanks!
I'm finally getting the idea. Nylon (like CoatsD-64) or (better still) Polyester 92 is what I'll be using on all my seams ... for strength.
Then, for looks and because I like the way a fatter thread appears -- and also for the UV protection factor -- I'll be using something like the Polyester size 92 on all French- or Top-stitching, which is mainly there for visual design and also what happens to get beat on by sunlight.
Sounds like the most economical approach would be to just use Polyester 92 for everything.