Depends on the scale. In 1/72, you wouldn't really see a well-sewn seam. A patch might show up as less-weathered fabric. Keep in mind that fabric was doped, though, which tends to hide the repairs. Look at period photos - do the patches show up?
In larger scales, you might simulate a patch with a piece of tissue laid down with Future. I haven't tried it, so test it on scrap first to see if it gives the effect you want.