I keep the frame level as I am using my leveling jacks. This prevents any undo strain on the frame, windows, windshield, slides, etc. Once level, I make sure that my rear tires are not off the ground and do not worry about my front tires.
My front and rear leveling jacks stabilize the coach since they are firmly attached to the frame. Putting blocks under my front or rear tires is not going to stabilize anything because of the spring suspension.
If having a tire suspended off the ground is going to cause a problem with the tread, then my spare tire is in trouble. :B