The theory tends to be that if a customer sees a shop-assistant sitting down then said customer thinks either a)the assistant is bored/grumpy and not going to be helpful, or b)assistant is bored/has nothing to do because the place is crap and gets no customers, so customer shouldn't even bother going near shop. I think that's true ONLY if the assistant is really unresponsive to the customer and is sitting there looking un-busy. You can sit and clean things or fold things or do files and still attract customers, in my opinion.