I don't like the idea behind the CCS. It's a measurement of a tool at rest, and that tool isn't used in a static manner. My 5wt Z-Axis is reported to be a 7.5wt on some CCS charts, but it works just fine with a 5wt GPX line for the nearly 4 years I've owned the rod. I've even used a 4wt GPX line with the rod. I've thought
about trying a 6wt line on the rod only because it was suggested by a guy that uses the same rod a lot, but it casts fine with the 5wt and I'm lazy.
If the rod is working well with the line weight designated on the rod, then enjoy.
CCS: Sounds okay until you start adding the variables, i.e, the caster, distance, weight of fly, leader, etc. My wife and I can use the same rod, but we don't use the same amount of force when moving the rod. At times, I'll use more force than my wife, and at other times (usually the wrong time), she'll apply much more power than needed. When she's using the Z-Axis, she knows she used the right amount of force when the line is snapped out of her left hand. That's often less force, and proper timing, and she always gives that smile that says, "I got it right".
There are probably rods out there that would work better with a line other than the one designated, but I've never owned one. There are some tournament rods that reportedly don't load well unless you have a LOT of line in the air, but I only buy rods meant for fishing. I suppose if you had a 5wt tournament rod, and wanted to use it on a small stream, then a heavier line might work better.
I own graphite and bamboo 4wt rods. I've never done it, but could guarantee you that a bag of pennies hung from the graphite rods would touch the floor on the bamboo rods. The bamboo flexes from here to tomorrow, but casts a 4wt line with beautiful loops.