I keep it on cause it looks more cool, and less plain with it on ;D
I dont think you legally need to have it after you have purchased it , only manufacturers and sellers would need to keep it on for obvious reasons, it would also need to be kept on if it was used in a place of employment .
Edit : It would also need to stay on if it was being used in a public demonstration.
It depends. On my PPL I removed the sticker because it was placed on the air cooling ribs which looks weird imho. On my ebayser the sticker wasn't very adhesive so I removed it before it fell off by itself. On my other 2 lasers the warning labels are still in place.