For example, if you buy a development card on turn 5, and then another one on turn 8, do you have to keep them distinguished?
When you play a card, if it was one you got more recently, it may be a clue that the other card is a victory point, since it hasn't been played. If you played the one you got a while ago, then there is no doubt what it is.
In all my games I've played, however, everyone just keeps them out in front of themselves, so it's not too hard to keep track if you pay attention.
I am building an AI for the game (to get some insights out of it, as well and possibly determine fairness of certain map configurations), and while this is a minor point, I would rather be technically correct when constructing the rules for the game, since I plan on using some degree of hand information to describe the board state for the AI.