As said, the last rows are usually held by the airlines for "special cases."
I have ALWAYS received the backrow when I buy a "bid for it" ticket on Priceline, although, I usually can end up changing my seat online.
No smell issues. However, my biggest issue is the inconsiderates who "line-up" for the lavatory.
If you are not in the aisle seat, it won't affect you, but if you have the aisle, you can count on being smacked, knocked, nudged and having a number of "butts" shoved into pretty much any and all portions of your exposed self (note to self, keep face averted toward the window

)