Far as I'm concerned, you don't need an inside, but you do need to finish off the edges of anything you build. Otherwise when you look at the item end-on there is no thickness to it, like there would be in real life. If I really going good I do about 4 or 5 extrudes and bevels on the edges that simulate the detail of a real fabric edge, the very last extrude goes inside, back down the garment, so it has an inside, but it only goes in what would be an half inch or so in full scale. People never think about this stuff when they start building clothing - I know I didn't. Just take a cylider and nip in the top and you have a skirt- but how about the edges, and the seams, not to mention the zipper, buttons, pockets, straps and folds!