Ah, I see what you were saying. Well, what we did was take the money out each night. I'm not sure what she did with the money...but we'd put the empty cash drawer out in plain sight, so anyone thinking about breaking in would know at least there was no money in the cash register.
Another spa I worked at simply used a small, metal, lock-box. It was kept open most of the time and locked at night, and kept below in a cabinet. I've always found cash registers to be overkill. Unless you're doing high volume, along with a wide array of retail items, you're right, you probably don't need one at all.