I launch for a lot of flyers. I have to giggle when they "tach" their engines. I watch, and the RPM jumps 500 or more RPM, back and forth. They seem satisfied, and signal for launch. So, what are they looking for? What RPM satisfies them? The tach reading tells not much more than my ears indicate. For some, a tach reading is only a ritual, and the results vary all over the place.
(I do have a tach, but I only use it when checking engines at home on the test bench,)