Not having a calibrated ear, I use a tach every time I set up a new engine / prop. I feel that it is important to know whether the rpm is meeting expectations and I just can't tell the difference between 8,000 and 9,000 rpm or whatever.

I have tried the Hanger 9 digital volt/tach and got good results from the volt meter, but the tach was unreliable and entirely too sensitive to background lighting. Later, I ordered the Fromeco TNC tach - it seems to read correctly no matter the lighting, the background, or the orientation. Like I said, I only use it a few times per year and then only at home when setting up a new engine.