Adjusting the valve lash is not super critical. As long as there is enough clearance to let the tappet/rod rotate as designed in operation you are good to go.
The only real danger is when the lash is to 'tight' and not allowing valves to seat. There is no real danger if you are loose a thou or two (or five). Might tap a bit... but won't hurt much.
The feeler gauge method fails when there is wear on the rocker. You need both the valve stem and rocker nose to be flat and perpendicular... the wear will increase the lash.
Setting lash cold is quick and easy. Screw down till contact is made, then loosen the adjuster as many flats as you need (I did the math in another post IIRC 2 flats for intake, 3 for exhaust). This is probably the best way to account for wear as you are dealing with actual contact not a feeler gauge that requires perpendicular mating surfaces. Many motors that don't give you a lot of clearance to get in with gauges (motorcycles in particular) use this method. I did with my 06 Boulevard. Works fine.
