Rarely appropriate, usually there’s a potential merge that would be preferred.
Great care must be taken when deleting tags. When tags are destroyed, it is not easily reversed. Even if a tag seems irrelevant or useless, be sure to use the search bar to look for similar tags first. If you find a lot of similar tags or if those tags represent a large number of scenes, then it may be better to merge than to delete. Merging is always preferred over deleting, and many of the same considerations apply. Easy candidates for deletion include performer/director names, but make sure these are also saved in the proper fields of all the scenes themselves before submitting your destroy request. Sometimes series titles are used as tags. These can also be easy deletions, but it can sometimes be better to merge it into a similar tag. In these situations, the series title probably doesn’t need to be added as an alias.