I took Step II in August and I'm very happy that I did, for several reasons. First of all, you get it out of the way before interview season (not to mention the various holiday seasons). Secondly, if you did not do as well as you liked on Step 1 but feel you will do much better on Step 2, August scores will be available by the time you start interviewing and you can point out your improved score to the programs you are applying to. Finally, 4th year is a pleasure when you don't have Step 2 hanging over your head!!!
However, don't take it if you don't feel ready; it's better to be well prepared and pass rather than taking it without adequate study just to get it over with (and possibly having to re-take it as an intern - who needs that??)