Correct tappet clearance is important and relatively simple to adjust.
A mechanic once said to me a loose tappet is better than one that's too tight, so I suppose from that theory, slight tappet noise is acceptable.... However, I prefer mine to be as close as possible to manufacturer's specifications.
I must also point out at this stage the recommended clearances of 10 thou for inlet and 20 thou for exhaust is only for standard cam shaft. If you have an aftermarket high lift cam you will need to check clearances - if new cam the specs will be included in package.
So first up, run engine until up to operating temperature, then turn off and remove Positive Crankcase Ventilation (PCV) valve and if an automatic car then tie gear change cables back away from top of rocker cover.
Now remove rocker cover carefully so as not to wreck gasket, you should have a new one anyway just in case. Start engine and allow to idle, don't rev it with rocker cover off, you will spray oil everywhere. Select a 10 thou feeler gauge and a 20 thou.
As we have a 12 port head on our slants with 6 exhaust and 6 inlet ports, it is very simple to know which rocker you are on as it will be directly in-line with exhaust or inlet port. From front (the radiator end) of motor the rockers are.... Exhaust, inlet, exhaust, inlet, exhaust, inlet, inlet, exhaust, inlet, exhaust, inlet, exhaust.
Run the 10 thou through inlet rockers and there should be a drag as it passes through, if no drag tighten, and if too much drag back it off. Now run through exhaust in same manner but with 20 thou feeler gauge. If they have been out of adjustment you will notice just how much smoother the car idles.
Replace rocker cover with existing gasket if still good... or replace with a new one.
Do not over tighten bolts, just nip them up and check every couple of weeks and tighten if necessary.
Replace PCV and untie cables.
IT'S THAT SIMPLE.