You can either put payments into the register or the safe. If it goes into the safe, it's nearly impossible to steal because there's a time lock preventing the

You can either put payments into the register or the safe. If it goes into the safe, it's nearly impossible to steal because there's a time lock preventing the cashier from accessing it. But you can't make change. This means you have a optimization problem to have the minimum possible cash in the register to meet all change needs. Eliminating denominations makes the optimization problem easier, if nothing else.