I'm not sure I'm comfortable with the implied assertion that the Saturn V was the product of senator meddling (absolutely no argument with the shuttle however). The Saturn V was a von Braun rocket, and as far as I am aware was designed with a similar mindset as the earlier Saturn and Jupiter rockets. (It is my understanding that) they basically made it as good as they could as fast as they could, damn other considerations.
So basically the Saturn V used LOX/RP-1 in the first stage but LOX/LH2 in the second and third because that was the best configuration, but SpaceX is using LOX/RP-1 in all of them because it's simpler but it gets the job done. (note that SpaceX is apparently considering developing LOX/LH2 upper stage engine for heavier loads: http://en.wikipedia.org/wiki/Raptor_(rocket_stage) )
So basically the Saturn V used LOX/RP-1 in the first stage but LOX/LH2 in the second and third because that was the best configuration, but SpaceX is using LOX/RP-1 in all of them because it's simpler but it gets the job done. (note that SpaceX is apparently considering developing LOX/LH2 upper stage engine for heavier loads: http://en.wikipedia.org/wiki/Raptor_(rocket_stage) )