When I used to do the Christmas Post as a student, (many, many years ago) we were told to deface any unfranked stamps with a Biro before we put them thought the letter boxes.
Sometimes this still happens here but mostly they just arrive unfranked these days.
If you do that, you're best not mentioning it :-) - but I understood that with the volume of post going through the frankers at this time of year, some get stuck underneath others, and get missed.