Depends on how well you want the finish to look and the wood protected. Glassing allows you to smooth over imperfection, add details like panel lines, and give both structural and surface protection to the model.
You can paint over bare wood but you should use a sanding sealer over it first followed with a primer to help fill the wood grain and glue joints.
You need to be careful, both using glass or not glassing, to not add too much weight, especially to the rear of the plane and on the control surfaces. Surface sealers, primer, and top coats can add a lot of weight to areas that will affect the performance of the plane. This is why covering (Monokote) has been popular because you can keep the weight distributed evenly and keep the plane light. BTW, I have painted over covering with fair to good results but am not crazy about doing it again.