We NEVER taught cruiser ready that way. Shotguns are carried mag tube fully loaded, chamber empty, safety engaged whether in the rack or a scabbard/case.
Many racks don't hold the shotgun so securely that the action won't vibrate the action slightly open and give another avenue for debris to enter the action. As far as cases go, back in the bench seat days, agencies used to carry the shotguns in scabbards mounted in front of the seats. With the action unlocked there were instances where the action was brought to the rear as the shotgun was inserted, and then closed forward while being pulled out, resulting in Nd's.
I don't think there is any absolute in this matter, just our way of doing things.