yes, closing the petcock should be common practice after every ride.
the stock petcocks can and will allow gas to flow even in the closed position eventually.
Your best bet is to use an in-line secondary shut off valve as a "just in case" peice of protection, in conjunction with the stock tank petcock.
Or, if you have one of the carbs with a built in shut off valve, that will keep the stock petcock from flooding the engine when it starts to leak internally and allow gas to flow in the closed or open postion.
running 2 shut offs seems like overkill, but unless you put better petcock in the tank, it's the cheapest way to correct the problem. an in-line shut off valve(for lawnowers) is about $3.00.