Even for a flat tappet cam I would adjust the rockers static, without the engine running. TDC compression stroke on #1, back off the adjusting nut and let it "rest" for a bit for the lifter to relax, then run the nut down just finger tight to remove all lash from the pushrod, then set your lash the prescribed amount of pre-load. For a high performance engine I use 1/4 turn of pre-load, then tighten the set screw and give them a slight turn tighter to set the locks good. For a milder, more stock engine I'd lean to 1/2 turn of pre-load. Then turn the crank 90 degrees in direction of rotation and set the next one in your firing order. You can also do it by watching rocker action, rotating the engine until an exhaust valve just starts opening to adjust intakes one at a time; then rotate until each intake is fully open to adjust exhausts but I like the TDC method as it's straight forward.
If I were doing the old "loosen till it clicks, tighten till it's quiet method" (engine running) I'd want a set of old rocker covers with a slot cut above the adjusting nuts, as they tend to throw a lot of oil around as I recall. Haven't done one like that in about forty years myself.