I use this rule of thumb: whenever someone gives of their time in service to me or my goals, I always send a thank-you email. This goes for small things, like emailing the grad assistant who squeezed me in to see a school administrator when he would be otherwise unavailable to the administrator himself after the meeting. Every favor, recommendation, meeting, interview, offer, and bit of consideration gets a (sometimes very quick!) email of thanks. I have NEVER irritated people with this practice, in fact, I believe it has helped make me some pretty strong allies over the years.
I highly recommend everyone take up this practice! So many in higher ed are not thanked enough, those emails seem to mean a lot.