Valve Drop and Piston to Valve Clearance

If we are talking about PV with the cam already installed, I generally use a dial gauge. You can use feeler gauges between the rocker and valve stem, but that's another story.
One of the reasons I do it that way is because I like to advance and retard the cam up to 8 degrees and see what the clearances are at those numbers.
If I use clay, then I need to reset the clay every time I change the timing and do a check..Head off etc.
A dial gauge when used correctly is also more accurate, but a lot of low comp street engine have plenty of clearance anyway, so clay get's the job done.
That way later on if I feel the need to adjust the cam timing, of if the customer wants to, I know what clearances I have. Time is money, and customers like competent workmanship for a reasonable price ..