I have never seen valve lash get bigger due to normal wear. Valves wear into the seats and the valves get tighter. Heavy duty operation means driving them like we used to drive trucks. Wide open throttle all the time except when we needed to stop. This is why heavy duty operation also calls for a colder spark plug. Our GMCs with solid lifters had to be regularly adjusted to keep the exhaust valves especially, from getting too tight and burning valves. Adjusting them a little on the tight side just to quiet them down is probably OK if you really baby the engine and don't demand much ofit.
