I just bought 2 Harmony Transit backless boosters for my 8YO and 9YO to use in my FILs vehicle this summer. They are supposed to have a "seat saver" design and my FIL was sorried about the upholstery in his new vehicle. Our old spare backless boosters were expiring anyway. On page 19 of the manual it says to lock the shoulder belt portion of the seat belt when you are buckling up the kid. I thought that was frowned on now. It doesn't say it is optional. I don't see my kids or my FIL locking all of the belts every time they buckle up. Does that mean I can't use this booster?