I'm going to toss my two cents worth in also, though it might not be worth that much...
Let's say you're carburetor float/valve is perfect...the float shuts off flow, and at just the right level.... Now push the mower back to the garage. Every time you hit a bump, every time you push the handle down so you can turn it, every time you jostle it around in the garage, the float bounces around in the bowl letting a little more fuel in, fuel that you aren't burning since the engine isn't running. Sure, the jet lets in only a tiny flow...but for how many days until the level comes back down to the normal shut off level? Where does that extra fuel end up?
While drawing that gloomy picture, let's make the horror yet more nefarious, gentlemen...you're out to mow again and you decide you need to scrape that nasty brown grass cake from under the deck. So you turn your mower on its side and go after it with an evil scraper. Or you may feel the chaotic urge to sharpen that grass amputating blade under there. Or, heck, maybe you just want to check to see if you need to scrape or sharpen at all.
With any of these normal mower happenings you would be better off if you had run it out of gas the last time you shut it off.
