Can't hurt, but in my experience it didn't matter much.

My first one was with my ongoing therapist, and that was easy. My second one felt like a rubberstamp. A few basic questions, enough time to make it look legit, but they just wrote it.

By the time my wife got letters, she'd been transitioned for years and not established with anyone. Same thing, with both letters. They just saw someone doing ok and gave the go ahead. And she doesn't ever fem up, just like soft butch.

Like, hopefully if you've been referred to someone for a letter, they understand trans people a bit by now. But you know, it depends a lot on where you live and everyone is different.

Edit: oh, you posted another reply while i was writing this. In that case definitely fem up. If nothing else it might help you feel better in the session.