When a tire tread depth test is performed, does the tire, or is it best for tire to be properly inflated? Would the tire being underinflated affect the tire tread depth test results?
For the most part, it won't have any effect on it. The reason for this is steel belts which run the circumference of a radial tire (radial tires are found on almost all passenger vehicles ... bias ply may be used for specialized vehicles/needs, but really aren't too prevalent in day to day life). The steel belts help the tire roll, but they also provide a fairly solid surface as well, which means there isn't going to be much flex in the tread area.