When I built by first twin engine plane I just knew I had to have a tach. That is common sense. WRONG!!! First few flights I used it, but it has been gathering dust for years now. There was just too much trouble using it. All that you already mentioned. I tune it by ear. I go for reliability first, last, and always. Slight difference in RPM is not an issue. A reliable idle is better than one set to an RPM.