I'm sure it varies. In my 95, the Tweecer always read the same as the dash, but they might be connected, I'm not sure. Our 93 GT had a tach when we bought it and it reads withing 100 rpm or so of the dash.
I think the inaccuracy lies in the gauge itself. The EEC is nuts-on with its reading - it has to be for the minute (tiny, not a reference to time) changes it makes.
In my experience, the tachs tend to be off by a percentage. So they read off a couple hundred RPM at 3K and perhaps 400 RPM off at redline (as a random example).
I've noted my fox's tach to be off by 200 RPM at 3K RPM. The tach on the 94 is off by 100 RPM or so at 3K (I can't interpolate the stock tach further). I'm sure everyones' mileage varies on how far off they are.
Paul's method has always sat fine with me. Consistency is key.
My old 94 vert was dead on perfect.
My current 94 coupe is 300 - 400 rpm off throughout the whole rev range.
but previous owner has put a white face guage setup on and might have messed up the needle.