[JohnC]

+1

As the valve train wears the clearance decreases. As the clearance decreases the percentage of time the valve is closed decreases. As the time the valve is closed decreases the operating temperature of the valve increases. As the valve's temperature increases the propensity to stick increases. If the valve sticks you will be looking at replacing the piston and valve at the very least.

So, adjust those valves early and often!