When I eat a piece of fruit, I take the PLU sticker from the fruit and stick it on one of the days of the month. No rhyme, nor reason. I just like to do it.
As a side note, a Gala apple is 4133, a tomato is 3131 and Cutie stickers are just, well, cute.
I must admit that having a sticker makes the fruit more appealing at time of selection and purchase. A sticker-less fruit is only selected as a last resort.