I would say Definately AFTER all cores, but....
It also depends on how strong is your Step 1 score. If you got 240+ on Step 1, I would delay taking Step 2, UNLESS you are sure that you will be able to get a Step 2 score above or equal to that of your Step 1.
The above is fine if you are going for IM et al.. But for the more competetive specialities, a FMG looks more attractive to a PD if it ("it" here is on purpose, and not due to grammatical error) took all the Steps before applying.
Good Luck.