Well the best thing in my opinion is to play a variety of old and new. If you play all new stuff, people might not know the tunes your playing. If you play too many old stuff, then it gets boring after a while and people wont be able to hear new stuff and find the tune afterwards and think "this is what he or she played that night at the club..". Mixing it up a bit is good to me, and lots of big DJs play old stuff and new stuff.