However, some critically judged contests make it clear that the entire model will be judged, including examination of the undercarriage/underside. In those competitions, it matters greatly and untreated ejector pin marks, mold lines, parting lines, and copyright engraving are deal-breakers.
If one is building with a plan to never show or compete, no problem. One just shouldn't take pix of the underside when one is bragging about his world-class best ever build.