Leaderboard

Strictly speaking it is considered a mistake when it's not part of the melody voice. Since a fugue is mainly based on double counterpoint, you shouldn't use them at all. I'm not sure there is a good explanation for this, it's just part of the way people perceived music during the baroque period. But the point is: If that interval is not an essential part of your compositional idea (if it's not part of the melody voice), there's really no good reason to use it either, it will just sound very exposed and probably make your music sound worse than without it.