The use of a follow-up email is an interesting one. Going to an email may be less risky, in that they used an email to get the original confirmation so the contact person at least has control of that ..as compared with going in a card to the address on file...which as has been pointed out might alert a spouse to something that was not right. ...though I imagine a couple on a secret getaway may not agree to be photographed anyway.
Some of the guest managment programs offer automatic email thank you emails to go out upon checkout or a certain number of days after checkout.
I have chosen not to use it because I prefered to customise the thank you by including details about their stay (that would make them know it was not generic). But to be honest...I often fall behind on them... so then the question becomes...is a generic auto email thank you, better or worse than no email?.