From the SRD:
Initiative determines the order of turns during combat. When combat
starts, every participant makes a Dexterity check to determine their
place in the initiative order. The GM makes one roll for an entire
group of identical creatures, so each member of the group acts at the
The GM ranks the combatants in order from the one with the highest
Dexterity check total to the one with the lowest. This is the order
(called the initiative order) in which they act during each round. The
initiative order remains the same from round to round.
If a tie occurs, the GM decides the order among tied GM-controlled
creatures, and the players decide the order among their tied
characters. The GM can decide the order if the tie is between a
monster and a player character. Optionally, the GM can have the tied
characters and monsters each roll a d20 to determine the order,
highest roll going first.
Nowhere does it state that the GM must reveal what he rolled for monsters, as such it can be left up to GM discretion.
Also note that it doesn't state that the players need to reveal their initiative roll to other players. They only need to let the GM know so he can rank them in order.
I've played in games where players initiative checks are secret because we don't know what order people are going to go in. It can make the first round of combat more interesting as people have to make choices without complete information.