almost every light duty truck engine I have ever had apart, had valve rotators either in the valve retainer or spring seat, so the exhaust valves would rotate to keep carbon from sticking to the valves and seats. I would think the rotation of the valves without the special rotators, which are just thin roller bearings, would have to do with how much seat pressure you had and how the retainers fit the springs, and the surface the springs sit on. A good hard slick valve cup like a lot of aluminum heads have, I would think would make it easy for the valves to rotate. I have never heard of valve rotation as a sign of valve float. I see lots of valve cups on used heads with signs of spring rotation, and I know most of those engines had no issues with valve float.