I guess this is something I should have done years ago, but was never sufficiently organized. I tested a dozen of my glazes at cones 4, 5, and 6 to see which worked equally well across the range and which were clearly better at one end or the other. Most were either OK across the range or better at cone 6. The exception was the pink glaze I make that's a mix of Burgundy Opulence and a cone 6 majolica. It began to fade to white where thin at cone 6. Here are some test tiles that show three of the glazes that definitely worked better at cone 6.
The glazes are from left to right: Zakin's Mouse Black, Pinnell Bronze Green, and Blue Sugar--from Clayart several years ago. Cone 4 tests are at the top and cone 6 at the bottom. Blue sugar--which really needs to be on a darker clay body--does not get its "sugar" until cone 6 and mouse black doesn't really melt until cone 6. Bronze green is decidedly smoother at cone 6. The shine on the glazes doesn't show in the images, but mouse black is glossy with surface texture and bronze green is more of a satin that truly glossy.
The mouse black differs from the original published recipe, which called for Albany slip. I subbed Alberta slip and then added enough gherstly borate so that it would melt at cone 6.