The way I see the world, it is noticeable not to acknowledge someone acting on your A2A. Human beings function that way. Quora may think they are above human sociability (which is why they keep denying that this is also a social site). But Quora users are not.
I don’t obsess about every upvote, but I will certainly notice it if noone upvotes an answer I’ve given—including the A2A’er. And I will read something in to that.
What form should the acknowledgement take? Well, there are multiple forms, and there is widespread confusion about exactly what the Thank button is for (How many of you use Quora’s “Thank” function after reading a good answer? Is sending “Thanks” without upvoting kind of like a backhanded compliment on Quora? What is the difference between sending “Thanks” and agreeing with/upvoting an answer on Quora? These features seem kind of similar.) My own behaviour, like Edward Conway’s, is to reserve Thanks for very good answers.