Router Base Smoothness
One test piece of veneer showed that some glossy areas had been buffed into the surface after trimming the edge with my router. Upon examination, I discovered that the plastic base of the router wasn't perfectly smooth!
The mounting screw holes on the base had small raised ridges that buffed the veneer surface. This caused glossy areas, visible when tilting the test piece in the light, down the length of the trimmed side. If I didn't catch this, I would have had to sand the thin veneer even more to remove the affected areas. Veneer is very thin, and can't take too much sanding before you go through it.
After discovering that my router base was the culprit, the fix was easy. I took a sanding sponge and sanded the bottom of the router smooth. No more raised plastic ridges around the screw holes, and a test trim on veneer came out perfect.