Funny this came up now, I just came in and got a shower after breaking in my new SK .91. I did pull out my Tach but just to see what RPM I was pulling with the prop I have on. For most two strokes I don't bother using one unless there is a problem and I want to dial it in and know where I have the rpms at that setting and prop. Four strokes I almost always use one now, I run several YS engines and being older models you can't really hear the rpms rise and fall. Gas engines I use the tach too but not too often after they are set.
I have had several makes of tachs and most of them are junk. I broke down and spent the money on a TNC because I tune so many different engines I wanted a good one. I have tested other tachs against mine and found one taCH THAT HAS BEEN DEAD ON AND NOT SO MUCH MONEY. iT'S THE COMBO TACH AND VOLT METER SOLD BY hORIZON hOBBIES. My thinking on this is you need a volt meter so for the price you can get a good meter and tach all rolled into one. I could have saved myself a lot of money if I knew this was a good system but what the heck, you need a good tach to test the others against.