i respond only if the invitation tells me (i.e. to email possible dates, to call the office to set up a date) or to reschedule a date that i can't make. otherwise, i don't say or do anything. i do, however, write thank you notes to my interviewers after the fact. but that's just me. others may tell you differently. try not to overthink this process too much, or it'll drive you crazy (or crazier, as i've learned). i know it's easier said than done, though...