I guess it depends. This is something I have done at past jobs, multiple times, and it did not work out well.. at all. Ever.
The "what if you break up" part is a very serious consideration. I'll tell you what generally happens, one person ends up quitting their job. So, yeah, you can look at it from the "why let work interfere with potential happiness?" perspective, but you can also look at it like: "why let *potential* hapiness interfere with gainful employment?" It sounds like it's not an important job to you, but it may be to her.
Now, that's serious relationships. If you're just talking about some random sex with coworkers (hey, I've done that too!), this can get ugly in a different way. First off, if you work in a place that is predominately young -- teens to early 20s -- be prepared for the worst kind of gossip imagineable. Also be prepared for other women you didn't even consider suddenly not liking you so much anymore due to jeleousy. The environment can be very much like high school. You don't realize it until you cross that barrier, and then all sorts of stuff you didn't expect comes out and you regret it.
It's true people spend a lot of time at their job, but they tend to do so and be thrown together not because of an honest desire to be there and hang with like-minded people, but to make cash. Most people hate half the people they work with. As such, you can find yourself easily mixed up with someone who you otherwise probably wouldn't date -- and for good reason, generally speaking. I mean, do you have anything in common but working for the same store? Maybe, but maybe not. A lot of time co-worker interest is purely physical. If that's the case here, don't think of it as potential happiness.
So, I'd respect her self-imposed rule, honestly. She may have dated someone she works with before and just plain knows better now. If you ever hop jobs, grab her phone number on the way out.
