I don't bother with A tach with smaller two strokes for just sport flying unless it's for something special like racing or I'm really having problems with it.
I do use the heck out of mine on four stroke and gas engines.
I have three of them under A bench that are useless, one very old one and two that everyone has seen advertised and probably have one. I had to finally break down and spend the money on A TNC for all the four strokes I was tuning, sweet!!! Is it worth the money?? Only when I really need it.