I wouldn't rely on them, though the ones in the a/c I fly seem to be quite accurate.
In any event, if the gauge says you have 4 gals left (20 mins in a 182) are you really going to push it another quarter hour?
Wasn't there a case in England recently where a doctor put a twin into a house because it ran dry? It was a rental and the owners wouldn't allow full fuel between renters.