At least in these here parts whar summer is most of the year and bikini makers are glad of it, it is a good idea to shut off the fuel. I have seen fuel tanks nearly emptied by an interesting pumping action. Seems that, on some bikes, when the fuel warms up and expands in the floatbowls, some squirts up the needle jet and collects in the throat of the carb. It then evaporates, cooling the floatbowls so the gas shrinks. Of course, this drops the floats and the bowls fill up. The fresh fuel expands, squirts up the jets, and the cycle starts again. Some 20 years ago when bikes were parked side to side for a whole block in front of The Nest (and later the pizza shop) on most afternoons, you could tell by the raw fuel smell just who had forgotten to close the valve. Can't necessarily say petcock for that bunch because there were some classic bikes like my Panhead that had a screw knob on top of the tank to shut of the flow.
