This will largely depend on what type of finish you want to have. You can paint on top of old paint but often this won't look as good as painting on the surface directly and also the paint won't adhere to it as well as it otherwise wood. If you want the best possible results then it's always the best idea to strip the paint back as much as you can. So in answer to the question, you don't have to degloss but it is advised is you want your surfaces to look as good as they can.