I make it a personal point to take off the previous sticker, clean the area where the previous sticker was to remove any left over residue using alcohol and a rag, and then put on the new sticker. The stickers we use simply say "restring date" and then 4 months from the time of purchase.
Is this marketing, a way to make sure people come back in? Sure, but it's also based upon the 3 times a week average that most people in our area play. Also I always make sure to put the sticker in either a completely blank space or over something that doesn't need to be seen.
"I knit with fence wire for a living" .... its as exciting as it sounds.