I have a grey card or two. I do not find them that useful. Either for exposure or colour setting.
Most of us have our own preferences. And if you shoot raw, what is the truth anyway?
If I am processing a bunch of images I set the first to taste then use the same setting for the others.
If you use the dropper tool on the grey. Card in the image. You will find readings vary over the surface.
I tend to look for a grey in the image and bring that to neutral. I then adjust from there. A true neutral usually looks. Far too cold.