Normally, Mediawiki convention is that an article should be assigned to the most relevant subcategory and shouldn't also be assigned to the subcategory's parent category. An exception to this is the case of "non-diffusing subcategories", as per MediaWiki/Wikipedia terminology.
The "Notation" category is a subcategory of Tablature but can be considered to be a "non-diffusing subcategory", as "Notation" doesn't define a type of Tablature, rather it indicates a special subset of articles within the larger set of Tablature articles. Therefore, I think sheet music articles, which have been assigned to Notation, should also be assigned to Tablature. With hindsight, the cat/subcat structure should probably have been something like this (top to bottom):
Category: Notation (or Score, or something wildly generic covereing all types of musical notation) Subcategory: Tablature Subcategory: Sheet music Subcategory: Some other musical notation system that I can't think of right now...
However, I don't suggest changing what we have now, just pointing out that the current categorisation of tab and sheet music isn't ideal, which is why I think it helpful to assign sheet music to both the Tablature Cat and the Notation Subcat. FWIW, YMMV, etc, etc.