Valve clearances are adjusted when necessary, such as when replacing valvetrain components or if there's noticeable noise. To check, listen to the engine; if noisy, adjustment is needed. This should be done with a cold engine. Remove the valve cover, position the number one piston at Top Dead Center (TDC), and adjust the number one cylinder's valves, starting with the intake valve clearance. Use a feeler gauge for correct thickness. After adjusting, tighten the locknut. Repeat for other valves. Rotate the crankshaft to adjust valves for other cylinders, then reinstall the valve cover.