if you do a search, you will find some previous threads about this too.
really, it depends on the person. if your step 1 score was strong (this means different things to different people) and competitive for the field that you are applying for, you can hold off and take it later in the year. if you want to improve on your step 1 score and have residency programs see it before they offer you an interview, i would try to take it before september, because that's when people start submitting applications. Remember too, it may take up to 6 weeks to get your score back, although it seems like most people get it back in 4 weeks.
i have heard things like some programs want to see that you passed step 2 ck before they rank you but i didn't run into that during my interview season.
i would schedule step 2 cs now, even for next winter because the dates fill up very quickly. you don't want to be taking it next april/may.