#1 There is no law stating that the pumps must state a Cetane rating. I don't know if a retailor would be in violation of any laws if their pump said a level which was higher than the min and yet the fuel met the min or was somewhere in between min and advertised.
#2 By law ULSD cannot fall below 40
#3 Cetane levels vary between batches of fuel.
Now...your question: "In short, the higher the cetane number the more easily the fuel will combust in a compression setting (such as a diesel engine). The characteristic diesel "knock" occurs when the first portion of fuel that has been injected into the cylinder suddenly ignites after an initial delay (once ignition occurs, all the remaining fuel burns smoothly as it leaves the injector nozzle). Minimizing this delay results in less unburned fuel in the cylinder at the beginning and less intense knock.
Therefore higher-cetane fuel usually causes an engine to run more smoothly and quietly. This does not necessarily translate into greater efficiency, although it may in certain engines."
http://en.wikipedia.org/wiki/Cetane_number