You don't have to keep your rookie picks but you do have to pick 'em (or the robot will). Once all rookies are picked then you can drop them to the FA draft just like any other player.

As for value of the picks, there is a BIG difference between real time and trad leagues. I would suggest going to another real time league and see what the FA picks were for last year. This will give you an idea of what the picks are worth.