I don't expect any thank notes when I subscribe, and it took me awhile to even realize I had a wall with those. Now, when I do receive a thank note, I generally ignore it unless:
1. It brings something new to the comics I'm following, a new perspective / angle or some bonus content. As I subscribed, to me it means that I like the author creativity or universe, therefore I'd only notice a thank note that would somehow display the same creativity.
2. It makes me feel considered by the author. So a genuine thank with a personal touch, and not a standard note.
With this in mind, considering that I haven't found a creative way to thank yet, and that I'm unable to convey any genuine feeling, I haven't sent thanks note so far (even though I do truly cherish all my subscribers, I love them all!). If one day I have a brilliant idea, I might start doing so.