My therapist did ask, but in the context of exploring what I wanted from her. I thought that was reasonable, because she made it clear that the answer didn't affect her willingness to treat me. When I said I probably did want GRS eventually, she said to let her know when she'd need to write the letter. (Same thing for HRT; she said I was clearly stable and well-adjusted and she'd write whatever I wanted, whenever I needed it.) I was lucky to get one like that on the first try, but I was willing to walk if I got a therapist who tried to thwart me.
She also basically let me stop coming after the first year, so long as I checked in occasionally until I needed the letter. Once I'd had GRS, I was done with therapy too.
I liked this method, really. It gave me some support if I needed it, but for the most part I was "informed consent in actuality if not on paper" : my endo also never bothered to read the letter, and my GRS prep consisted of reading the required paperwork on potential complications, etc. *very* carefully.