I asked same question to my t, and he told me that if I thought I want to tell that to someone, it is relevant to my current feeling, or something is bothering me, all of that means it's worth talking in therapy. I often find something I did not think important means something to my t, so tell as much as you can.

I had (and still have) a lot of trouble to talk in therapy, so I usually write down everything I wished someone could listen, or something bothered me, and bring it to next session.