If it's UCI's rule you're wondering about, here's what they say (Article 1.3.014):
“checks on the horizontality of saddles will be conducted at WorldTour events, World Cup events and World Championships for the road, the track and the cyclo-cross by measuring the angle of incline of the saddle, considering the plane passing through the highest points at the front and rear of the saddle. This angle must be less than 2.5 degrees with an error tolerance of 0.5 degree.”
http://www.uci.ch/mm/Document/News/R...NG_English.PDF
But as noted above, only people competing in UCI sanctioned events should feel bound by this rule. For the rest of us, a level saddle is a starting point only, and can be adjusted as needed for rider comfort or preference.