My thought's are that a tach should be within eye site without turning the head. You must keep your head straight ahead to insure control. I saw a couple of guys had built a mount that fit around the dash pad to place the tach where you want it without destroying the dash pad. It has been many years since I've seen it done but that is great. Way too many use to just screw them to the dash pad with no thought of what was to be in a few years. Another problem is that placement in some positions causes the tach to be hidden by the spokes of the steering wheel which leaves you to where you might as well not have one. Honestly, the tach could be hidden from normal view but put a shift light in major view.
Just thinking out loud.