I use a tachometer to "perfect" my tuning, whether for idle or high speed needle adjustement.
I only do this once and when I change plug, propeller, or fuel.
It is not something I do commonly on the field, I rely mostly on my ear for these small weather related adjustments

For Idle:
I set my engine in such a way to pick up instantaneously when transiting to full throttle; and for this I need to let in the correct amount of fuel/air mixture that will be burnt completely at idle without leaving any accumulated fuel in the piston head. I am not afread of lean mixtures, bcs at idle the engine fins are able to cool it properly without any extra help
When I set the idle, let's say at 2500rpm on my OS 46AX MAX, I then "clean" it by doing some full throttle revs, and then drop it back to idle and watch the speed with a tacho.
If the engine idle is set accurately, the tachometer should read 2500rpm all the time with minimal variations, and you can idle forever until the tank is empty. What you will often notice is that as you idle, the rpm tends to drop when the needle is not set correctly and it eventually dies!
Of course we are assuming that the tank to carb position is correct and the issue is only due to needle setting!

For Full Throttle:
I use the tachometer to detect the highest peak in RPM and then I drop it down towards the rich side by 500rpm. This depends a lot on the weather, and I often play with that setting and adjust it by ear after the initial tuning with the tacho.