Yes. Your gelcoat will come out in the same condition as your plug surface. You can leave it and try to clean up the mould, the issue is that not only is it generally easier to sand the male (plug) version rather than the female (mould) version of the part, but gelcoats are by design extremely tough. Sanding them is a bitch. One can get rid of small defects on the mould surface by sanding and polishing, but to try to correct larger or more frequent imperfections is a tedious task indeed. You also run the risk of exposing microporosity bellow the surface of the mould, depending on the tooling system you use.