I think it depends on your expectations surrounding the letter. It is important to really examine your motives here, do you expect a response? If so what kind?

Relationships are sad when they are end, but are they ever really over?

I did not give me therapist a letter or a card when i terminated - although in an effort to kill me, he and I have kept talking.

A picture of you conquering your fears would be very affirming to him I'm sure, but again, this remains, and always will about you.

In objective terms, aside from neediness etc... what would be best for you?