I have in the past put my trays out, but now the vast majorities of the hotels at which I stay, place little cards or otherwise indicate NOT to place your tray in the hall, but to call roomservice and have them pick it up. I prefer this, as I personally don't like passing down the hall of a nice 4 or 5 star property to have to look at last night's dinner or this morning's breakfast. I am sure it is mostly for aesthetics, but also to help control the pest population and the harboring of assorted vermin.

However, I have never slipped a note under someone's door about them leaving the tray outside, calling them a slob, etc. I think that would be management's job to let them know that leaving a tray outside is not something they'd like to have occurr.