During a discussion on this topic some years back I jotted down some data on an out of state road trip in my prior Lincoln. The results then, as now, show that the dashboard indicator is extremely accurate although I will not claim that every indicator on every car is that way. Note that the dash indicator does not break it down any further than tenths.
Home-Goodland, KS
393 miles - 15.1 gallons - actual mileage 26.026 - display 26.1
Goodland - Loveland, CO
226 miles - 8.34 gallons - actual mileage 27.098 - display 27.1
Loveland - Raton, NM
269 miles - 9.96 gallons - actual mileage 27.008 - display 27.0
Raton - Alva, OK
353 miles - 13.66 gallons - actual mileage 25.841 - display 25. 9
The final leg was not notated; it was midnight and I didn’t care at that point. 
Usually I don’t resort to jotting this stuff down and most figuring is just done off he top of the head, but in this case I just wanted to satisfy my curiosity and be as precise as possible. The only other instance of writing things down involved a road trip in which I used acetone mixed with gasoline to verify that bunk about fuel mileage increasing because of the use of acetone.
Anyhoo, I would say that the dashboard indicators are extremely accurate in my cars and as TwinTurbo mentioned; it kind of keeps me tuned in as to any potential performance drop offs that may not be noticeable.