Not ALWAYS required
The reason to check it is because you don't want the valves to hit the pistons.
If you're buying valve train parts (cam, timing gears, timing gear advance, lifters, pushrods, rockers, valves, valve springs, valve spring retainers) that have been proven in the past, you probably won't have a problem. You could still buy the correct parts, and have the timing gears advanced so far, that Mr. Piston meets Mr. Valve! Since there are so many variables in the valve train, it's a good idea to check it.
The "old school" method of checking piston to valve clearance, is with a lump of modeling clay. Stick it on top of the piston where the valves will be. Then assemble the head & install it on the block WITH a used head gasket. Now hand crank the engine over 2 complete revolutions (720 degrees). Then take the heads off and measure the thickness of the clay, at it's thinnest point. That is your piston to valve clearance. BUT remember that with juice lifters (hydraulic), there is a cusion of oil between the lifter plunger & the bottom of the lifter. If your lifters pump up, it will cause the valves to extend further into the bore & thusly be closer to the piston.
Now you get the idea why cams & valve trains are like so much smoke & mirrors!!!!!!
68COUGAR