The majority of dioramas that you’ll see at a model exhibition are uncovered. The scene is constructed on a flat base and there is no case over it.
Museum dioramas, on the other hand, are usually covered. The primary reason for this is naturally to prevent damage to the work, but there are other benefits as well. A well executed display case can add an extra dimension to the exhibit being displayed. These lighthouse exhibits in the Estonian Maritime Museum are a good example.
Rather than covering the lighthouses with standard rectangular cases, custom made cylindrical glass cases were used. These conform much better to the shape of the subject. The exhibits are carefully lit from below to minimize distracting reflections that can sometimes be a problem with curved glass. These cases were undoubtedly more expensive than plain rectangular cases would have been, but the aesthetic payoff is undeniable.
If you haven’t yet tried covering your diorama with a display case, consider it for your next project. It will help give your finished scene that museum look.