This is a really interesting/complex topic. It really depends on what you consider your library, and how important collecting is to you, and to some extent your history.
Personally, I just add to library freely when I see something that I might like. Then later, I sort by date added/filter by not played and I can see a todo list in the album browser.
Sometimes I see stuff in there that I want to delete, and I do.
But–I don’t mind having excess stuff in my library that I don’t care about, because for me library inclusion is much more about accessibility than it is about collecting. I don’t feel total ownership over every piece of content in here. It is kind of like a personal streaming service–lots of content that I can access, but not all of which I feel ownership over.
For the stuff that I really own in an emotional sense, I try to favorite it. Our product doesn’t do a perfect job at encouraging favoriting, so I feel like I am swimming upstream when I do it, but I think it’s the right place, long term, for me to keep track of that stuff.
One of the annoying gaps with favorites (and tags, too) is that we don’t support favoriting or tagging out-of-library content. This is a historical detail…I wouldn’t design the product that way today. We’ll probably lift those restrictions at some point. If we supported tags out of library, you’d have a really easy todo list solution.