by Wheelockian
I've always assumed that since it's not mentioned in the rules, the information is public. You can already see the armies, so why not the cards? Sometimes, in the process of trying decide what moves to make, a player will ask another what cards he or she has, and the other person obliges--all part of the decision making process.
On the other hand, official ruling or not, if your group prefers to play with the cards secret, I don't see why you shouldn't.