I also transported (street)bikes for years using tie-down straps. When I got my dirtbike, a salesman advised that I use a plastic spacer. He said that the difference between riding and transporting is that when riding, springs are only compressed for a second or less, and then rebound. The amount of time they are compressed when transporting can shorten them and change their spring rate. So I bought one.
Well, when I cranked down on the straps, the spacer smashed down the top of my tire. Then on the trip from my house to the track, the spacer popped out, probably from all the small bumps in the road, which left my bike with no tie-down tension, which resulted in my bike swinging back and forth hitting the other 2 bikes in my truck.
Spacer returned. Back to straps.