My site does encourage this for those kids who are unable to otherwise remain properly positioned. It's an option. The vehicle manufacturers don't give a definitive answer because the Emergency Locking Retractor will engage regardless & the Switchable/Locking Retractors frequently become accidentally engaged even by adults. It's not a safety risk to do so, it may merely be uncomfortable for the passenger. If a child is apt to lean out, around or forward while in a booster seat, then engaging the shoulder belt lock usually serves as a reminder to help keep them properly positioned. A child must essentially remain as still as a crash test dummy for the booster + seatbelt to offer full protection. I taught my child to always engage the locking shoulder belt because it prevents her from forgetting to sit still (& this is a child who is more educated on vehicle safety than most adults: kids too often simply have short attention spans & get caught in their own minds, forgetting to be "perfectly" still for the entire length of every ride). If you feel like engaging the shoulder belt lock will help keep your kids properly positioned, then go ahead & do it. It does not increase their risk of injury to do so, but does hold the potential to help reduce their risk. Some kids only need to be reminded that if they move lean around (or put the shoulder belt under the arm or behind the back) then the consequence will be to lock the belt. Some kids hate having it locked, so the mere threat is enough. Some kids prefer to feel more snug as they feel safer. Either option is fine so long as neither the carseat or vehicle manual actually forbids it (none do that I know of)