I've made it a habit to always thank the Question Asker if they select my answer as Most Helpful. I'm aware that some people just select any answer as Most Helpful so that GirlsAskGuys will stop sending them those annoying messages (no offence) lol. But if someone actually took the time to consider all the answers they received and chose my one as Most Helpful then it would be common courtesy to thank them. Similarly, if someone thanked me for choosing their answer as Most Helpful then I would I would say "you're welcome" or "no problem" just out of politeness.
I mostly do. The only time when I don't is when I'm the only answerer, so the QA doesn't need to choose anyway hahaha.. But I sometimes don't reply after the MH thanked me. It depends on the answer. If it really was the most helpful, then I'd state that they deserved it. If it were just because I had to choose one and I picked the one with the longest answer (more effort given), then I don't reply. Unless I've talked with the user before, in that case it's always nice to say "you're welcome" or something.
Mommy and daddy have taught me to thank people for their nice words, gestures or actions towards me. They didn't advise me about "Most Helpful Opinion" on GAG, but I'm smart enough to know it's still on the list. So, yes, I do.
I usually try to say thank you. It doesn't happen often enough for me to be qualified to give a proper answer ;) I usually don't follow up the thank you. That just draws the conversation on, and you know how I hate long drawn out conversations leading to no where. Argh.
I always thank someone for selecting mine as MH, absolutely. I genuinely appreciate being selected, so why not just SAY that, you know? ^_^ I respond a lot too, most of the time MORE than the other person, haha!