I rely on these tachometers quite a bit in my own topmaking, and I think @Iacopo would say the same.
Yes I do, when they are tops for the longest possible spins, in my case.
Comparing the spin times is the simplest way for to evaluate the best tops but I prefer to use the tachometer, for to have more complete data and for to understand better the reasons that a top spins longer than another one. With a tachometer I can essentially evaluate four different parameters: the starting speed, the toppling down speed, the spin decay at high speed, (which is related mainly to the aerodynamics), and the spin decay at low speed, (related mainly to the tip friction).
These data of different tops allow for a reasoned comparison and help to improve the design of the top.
I am finishing a couple of new tops in these days, they are tops for longest spins, I will show them here soon.
It was a difficult year for me, (too much work), and I never stopped making tops for so many months before. I hope that this will not happen again because I love making tops, also I have some new ideas that I want to try.