i, like generic, took it in the middle of third year after one year of rotations. my CBSE score was 228 and my step score is 245. definitely my biochemistry, mole bio and some physio (those dang graphs!) were very rusty after a year of not using them, but i took extra time while studying to get reacquainted. while studying, i frequently noted i was able to retain clinically-related factoids (eg path, pharm, micro) more easily because i had a framework of clinical experience to build upon. so there's advantages and drawbacks. in the end, it kinda depends on your learning style and how heavily weighted the exam is towards clinically-applicable factoids vs not-so-applicable ones. i don't think biochem and mole bio were as heavily emphasized on my exam as usmleworld would have had me believe.
on the other hand, had i really been studying hard during basic science and knew my sh** cold i would have rather gotten it over with early. as it was, knowing i wouldn't have to take it until third year kinda led me to slack off during years 1.5 and i only really forced myself to study intensely for the 6 weeks before the step. then again, i felt that basic science was really rushed for me and i didn't get a chance to really sit down and learn things well the first time round (or maybe i was just a slacker) so i was glad for the extra time to apply basic science principles to clinical scenarios and learn them that way.
some of the previous posters have a point though. if i had taken the step before clinics, i would have probably performed better on my first two rotations (esp. medicine.) i definitely felt that my basic science knowledge was not solidified enough to let me shine on medicine. but after the first three rotations that handicap kinda resolved itself. maybe because i could use the info i learned on medicine for surgery and peds or maybe my later rotations were less dependent on basic science knowledge. who knows.
basically it's a toss up. you will probably adapt to whichever way you choose and perform well either way. for me, if i had to take the step right after basic science, i would have studied more during basic science to make sure i was prepared. even so, i don't think basic science prepared me that well for the step, so it was going to have to be a lot of self study anyway -- and for me, it didn't really matter when that self study took place, i'd be deficient at something or other. actually, if i had to study intensely for 4-6 weeks right after 2 weeks straight of final exams, i'd shoot myself.