Yes it's mandatory. The YS engines vreate a very high pressure in the tank as you would know.
When your carb is closed, the engine stops more due to no air getting into the carb anymore, but your idle needle is NOT all the way closed.
You could test this by screwing in the idle needle when the carb is close, althought I don't recommend it as the YS is a bit touchy to it's idle needle settings and you would want to mess it up.
Lot's of guys run a 3rd line with either a plug or a small plastic clamp that you can undo to release the pressure.