I'll send an email if:

• I still want to work there after interviewing
• it's to a peer (i.e. someone I might see in the future at a conference or another job)
• I really did enjoy talking with them
• they're on the hiring committee (as far as I can tell)

Then I'll send an email like: "hi, it was great talking with you, I hope things go well and we end up working together." Then if we do run across each other professionally in the future, we're more likely to remember each other.