I guess it depends 
I think there are different tags (file/study tags or categories, and notes tags), and for now I only used file/study tags. I use them to classify files in categories that can coexist. For example, a study can be about a single document (#article) or a topic (#topic), and both tags are inside a #range tag. Other categorization is about what brain area the study is about: #basal_ganglia, #hippocampus, #neocortex (all inside #brain_areas). Also I can classify files according to the research #status: #done, #to_do. And finally I use a simple star #(X) to mark important files and find them fast.
You also have the notes tags, but I didnāt use them much. I think they can be very useful to organize the reviews, as you can filter the cards to review by tags. By now I didnāt use this feature much so I cannot help