As a general rule, leaving the fuel valve open all the time is just flirting with disaster. That said, the sequence of whether or not the valve is closed before or after turning the ignition switch on/off, is less important than turning the valve OFF when the bike will sit for a period of time.
Let's be glad that our carb bikes have manually operated fuel valves & not those dang vacuum automatic valves (which I despise, based on experience).
