That's the problem with working with resident Role Sims, it becomes very challenging to make their lives seem realistic. And if they have a 24/7 role assignment, it's just about impossible to work with them.

I think these registers take on the hours of the lot on which they reside. So if you want a less than 24 hr cashier, for example, you would have to alter the Venues XML (if that's the correct one) so that lot it is on has a proper opening and closing time. Unfortunately we can't have two different cashiers on the same lot or lot type working different shifts within the same game as the XML is gamewide.

Maybe Arsil's custom cash registers might help?
http://modthesims.info/d/543967

Or maybe others here have come up with better solutions using all this newfangled fancy savvier seller stuff that I don't have yet.

I keep grappling with this myself, though. Not so much the 24/7 ones, those are hopeless, but even a resident Role Sim who has to work 12 or 18 hour shifts every day is really difficult to play around and pretend they have a life. But I just saw one recently who works a 12 hour shift every night that SP tried to push into a romance (they didn't get very far but it was fun to watch hands-off from a distance) so with activities like that not caste restricted, some things can be done with them. Ultimately I think I'm going to have rotate many of these residents out of these positions ever few sim weeks if I really want them to be an interesting part of my game though.