Unfortunately, they can make a cashier pay in Ontario if you have sole control over the register. They can't make you pay if you share a cash tray with other employees.
I worked in one store where each cashier had their own tray and no one else ever used it. If you went on a break, you signed out and took your tray and another cashier signed in with their tray to replace you. At that store, we were not held responsible for loses. I worked in another where we were held responsible and managers would frequently jump on your register when you went on break using your cash tray. As soon as they would do this, I would tell them that I was no longer responsible for any shortages and they would look at me shocked. The store where we were not responsible had better cash handling training and training against scams and better cash handling procedures than the store where cashiers were responsible for shortages.