I have been flying R/C for about 39 years. Never used or had a tach till I built a twin engine plane about 7 years ago. It seemed reasonable that a tach was needed for a twin. I bought the type that is light sensitive and could read the prop coming by. I never was happy with it because it was a bit of a struggle to get a reading with it. As I learned more about special features of R/C twins I used the tach less and less, and then just stopped using it. I tuned each engine individually, to run the best it could. I never tried to match the RPM. The engines were identical and the same age (purchased for the twin). I still have the tach but it is stored, with the battery removed. I just tune the engine by ear and sight and performance. I always tune for maximum reliability rather than maximum RPM.