The primary purpose of the glass is to provide a stable base for paint. A line (if for example, you sanded too much over a spar or rib) of exposed area might allow the developement of a crack in the paint over a long period of time. Covering it with a new layer of glass would prevent it from becoming a problem.
If the spot is small, I would just brush a little epoxy over it. If it was much larger than say a nickle, or a thinner longer line over 2" or so, then I might add a small patch of glass. I can't say these size guidelines are right or wrong, it's just what I would do.

Scott