Bruce thanks for the info. I talked to a buddy as to why they change the tires and shocks on their pro stock cars. You are correct in it is to help save tires but it is not a tread/traction issue it is to save the sidewalls. Cars are parked on the tires at the race event only and on pit mats. They attempt to keep the tire as clean as possible. He also said that the floor in his trailer is not a smooth surface and they have left cars in the trailer over night and have had a pattern in the morning in the tread area but have no data showing that as a problem in traction. He said some of the old timers do as you talked about and won't let the tire touch anything put pit mat, pit road and race track. They will change them out every night before loading the trailer as well. He said their used to be a team that had tires they would tow to the back of the lanes with and then switch out. They would go thru tech after the run and change out to tow back.
I think this is a real good question or topic of discussion. When I get in to work tomorrow I am going to send an email out to the tire companies to get their thoughts on trailer surfaces. I never really considered this an issue but if it can play a part in better consistency I will rip the diamond plate out.