I saw this post on Whip Up, called "Public Embroidery" about someone who leaves little embroidery tags in public spaces (which, frankly, begs the question when does public art become defacement of property).
I gave LaLa (age 4) a set of stickers that came with a scrapbook magazine offer. Later I bought her a set of St. Patrick's Day stickers from the grocery store.
Oh, and I think I have an answer of when it's "interesting" and when it's a problem: when the stickers end up on something important (like my kitchen table) as opposed to something of little value or something that's easy to un-sticker.
These are a few of the things I found in the house: