POS system in workgroup or domain?

I am looking into getting a POS system for my brother's restaurant since he is not knowledgeable about software/hardware. It is a small restaurant. I am looking at MS, but others also. My question is normally do you guys setup the terminals and server in a workgroup or domain? What is the norm on this. I was just testing a POS software from another company and i set it up in a workgroup. It mentioned mapping to the server from the workstation terminals(running XP) to run the database and services, printer spooler, etc. My problem is that if a savvy enough cashier can find out about the mapped drive to go into the folders and manipulate the POS software configuration. The only thing on the desktop is the icon to launch the POS program. However, the cashier can go into windows explorer and drill down to the POS software folders. It's hard to restrict the cashier. Thanks.