Dave Sochar brought up an excellent point in reply to “Who’s gonna see it?” The first impression is hugely important.
When you are delivering something like a kitchen cabinet set, all of the backs and sides are visible as the pieces come off the truck. If these look baggy and rough and your customer sees this, the first impression might not be too good.
Of course, you can say, "Those won't be seen after the cabinets are installed." But at that point, the customer is already nervous and, since they will not see the finished product for several days or even weeks, they may give more weight to these unseen areas than if they had inly seen the finished product.
I'm not saying that these areas should be finished to the same standard as the faces and visible interior surfaces, but they should at least be clean and presentable so as not to create a bad impression.