That's right Greggo. I looked at my manual for my 1968 SuperHawk and it says when parking for an extended period, turn off the petcock. My experience tells me an extended period is several hours or more. I'm sure all the 70s era CBs have the same comment.
I would never leave a motorcycle parked in a garage overnight with the petcock on. I've gotten out of bed to check on more than one occasion.
hate to highjack this thread, but since this is a safety issue, I'll say my piece and get off.
Not only can you get gas overflowing into the garage as stated, the gas can go into the crankcase if the intake valve on that cylinder is open. It can go by way of fumes which condense on the cold cylinder sleeve. Then washes down past the rings. In my mechanicing days, we did tuneups where we'd pull the drain plug and too much oil would come out (350s and 450s mostly). Gas in the oil.
I would turn off the gas when I'm pulling into my drive. That allows the engine to suck down the float levels a bit before i finally turn off the engine.
AS I stated elsewhere, modern bikes have vacuum operated petcocks where this is no longer an issue. Starting 1982 or so. On any manual petcock, turn off the gas when parking the bike, and try to use the centerstand for long period parking, to keep the floats from sticking.