I don't think one owes their T. That being said, if I were to leave my T, while I've told him I would just disappear (because I hate goodbyes), he's actually gone above and beyond for me and so if I were to find myself in a position to just terminate, I'd probably send him an email or a letter thanking him for his time and efforts.

I've worked with him for a good while though and he's told me that when it comes time for me to move on he'd like to still hear how I'm doing down the road (H and I have talked about moving). So. Y'know. Every relationship is a bit different. I think you find what works for you.