The rules state that when the game begins you should have somewhere between 40 to 60 cards in the set. I like to use 40 for two obvious reasons - it plays quicker and it's easier to remember the title cards.
When we play the first round, about 1/3rd get quickly identified, so quickly that sometimes we can't catch the name from the other team because it sounds so unfamiliar to our ears.
Do you find the game plays better to then review all the titles one by one after the completion of the first round so that everyone knows what they are up against or do you say "The hell with it. Let's just play!" Eventually, all the cards are identified but it may take several extra rounds for the more "clueless" clue-givers to know what's in the deck.
We always go through every card after Round 1. Time's Up tells you to do this in the rules.