Yes I absolutely always leave kudos and try to leave at least short comments when I can. I also use my AO3 bookmarks increasingly like a master rec list (I have over 400 fics there) and add my own tags to be able to find stuff better. Especially with fic imported from older archives the tags are often sparse so I add some.
I currently have 70 fanarts on AO3 (when did that happen???) and get kudos on older stuff which makes me really really happy. My few Stucky pieces definitely win at everything but I think that‘s because the fandom is so large. When I get kudos from a small fandom it makes me extra happy. :-)