I usually tried to expand on 15, then build another drone and put down a spawning pool. Against Terran and Protoss, this almost always worked (I was in gold league at the time). Even if they rush, it's fairly easy to hold off their attack. If the Protoss goes for a cannon rush or simply blocks your expansion with a pylon, things get a bit more tricky, but it's still possible to properly adjust to that.
Against zerg, I had the feeling that an early expansion doesn't work as well. If my opponent spots this, he typically killed me with his zergling output from one base.
Thus, when I was still playing zerg, I would expand first against Terran and Protoss and would do a 13 pool against zerg.
All the best,