When you say thank you, treat people with respect and courtesy it is good for business. I don't know about you but I always acknowledge every email I get. (From people I know...not spam!)
That's why I work 18 hours a day I think!
I just feel is anyone writes to say how much they enjoyed a presentation or wants to ask me someone it is pure common courtesy to reply.
Ignoring phone calls and emails from people you know or have met is like passing them in the street and just walking by. You just wouldn't do it