I have kept my parking receipts on my dash for an extended period after my trips to the beach. After a week or so the ink fades and you can't read them. So if you intend to keep them for possible ticket disputes in the future you need to place them in your glovebox or somewhere out of the sunlight. Also what the person said about people taking tickets off of another car so they won't have to pay for parking and it will look like they have already been ticketed is highly likely as well.