I think you'll find a fair few cashiers will send someone to the back of the queue for queue jumping if they spot it.
Queue jumping is of course a big no-no (in the UK).
that would be nice. I've never seen a cashier do that where I'm from, they just tend to ignore it and take whover approaches them. Of course, in the US you have to be aware that any customer you potentially get into a fight with has a gun, which is less of a factor in the UK.