Between hourlies, nothing UPS can do about it. Can create some messy situations if there's a breakup. I've seen one girl over a few years hook up with 3 different unloaders and after each switch while she wasn't an unloader it created tension between the other guys.
As far as hourly with supervision it is highly discouraged and UPS can do something about that. I've seen a sup and hourly hook up but they kept it so down low that no one knew until the hourly quit and then they made it public, and I've seen where sups are transferred to other shifts because it's became known.
My lovely lady works the same shift as me, and we've been together pretty much the whole time I worked at UPS and the whole time she has. As long as we're not working the same area our interaction is so minimal we're just coworkers and that's perfect. The bad part is anytime anything happens that regards her I get to hear about it from a bunch of people because apparently I can magically do something about it.