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.

I wouldn't think there is enough coinage in a register to be interesting as a theft target, but i don't know! I think they would still make bill change, but maybe I misunderstood, sure. Next time I see one I'll use cash to find out!