I understand how the "not friends" thing can hurt. I was recently talking with my T about how it felt almost like she and MC had unconditional love for me, which I felt I didn't get from my parents (yes, I have some maternal/paternal transference). After seeing my T's expression, I quickly added how I knew the T-client relationship was different, and she was like, "Yes, it's a different relationship," so I felt a little shut down. I sent her an e-mail right after the session, and she responded quickly and explained better what she'd meant, which helped me feel better. We then discussed briefly in the next session.

I also last week was a bit upset by my marriage counselor, who I felt cut me off in talking about something important to me and changed the subject (it's a bit more complicated than that). I sent him an e-mail the day after the appointment (my husband knew I was doing this), he responded and apologized, saying he hadn't meant it that way. Then in our session yesterday, he wanted to to discuss why I'd sent the e-mail instead of waiting till the next session. I said in therapy (with my T), I'd found that it worked better for me to express if I was angry or upset at the time, rather than letting it fester in my head. MC said maybe it's better if I sit with the anxiety (like till the next session), since that's something I have to work on.

Tomorrow I'm discussing with T the best approach--I saw the e-mailing/bringing it up at the time thing to be progress for me, as I've been using that approach the past 6 months or so, whether with my H, T, MC, or other people in my life, and T seemed to agree last time we discussed it.

So, to respond to your question, I feel like it's best to bring it up. Otherwise, it can kind of grow in your mind into something bigger than it originally was and lead to tension between you. When it could have just been quickly resolved and dealt with.