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Many HEMA people complain about the reverse grip trope in movies and video games in regards to swords. Most of the time they are right. However, lightsabers should be exempt from this criticism to a certain extent. Particularly, it should be acceptable in drawing the weapon.
Size
When worn, lightsabers are usually no bigger than a dagger. The reverse grip is widely accepted by most when it comes to dagger plays. This means that until a lightsaber is ignited and distance is made, lightsabers should be treated like daggers from the draw. Also, consider that most lightsabers attach to the belt vertically with the emitter facing downward allowing the reverse grip to be natural just like with the dagger.
Iai
There are several actions in iai (both iaido and iaijutsu) in which the katana is drawn in the reverse grip and this sometimes includes a slash from this grip. This is mostly because with Japanese sword etiquette, the sword is set on the floor on the right side.
Off-hand weapon
In some instances, a secondary lightsaber (often a shoto) is held in the off-hand in the reverse grip. As a shoto or kept deactivated this can be likened to the use of the dirk in conjunction with the Highland broadsword in which the dirk is used for some parries as well as close-combat thrusts.
In summary, people criticize the use of lightsabers in the reverse grip a bit too much. Unfortunately, the sequel trilogy ignored this in most instances of this grip and used it when it was not a good idea.