With a step 1 with a 90+, probably not AS important.
But the earlier you have it done, the more 'together' you look.
I got more comments about the fact that I already had my score in by interview season than I did on the score itself (which was 250+)
I'm curious about this, too. I haven't registered for CK yet and have heard differing advice from people. I did really well on Step 1 and am pretty sure that the odds are I'll do worse on CK, probably still good, but worse. Some people have told me I should take CK in January at the earliest because I can only hurt myself, but I might like to get it over with sometime in the fall. Thoughts?
Some programs (ie, UCSF) are now requiring a passing step 2 score before they will rank you. So you should check with the programs that you're applying for before scheduling a late step 2.
IMO, if you schedule your step 2 for after your initial application has been submitted, then you will probably get your interviews based on your step 1, and then even if you don't nail step 2, as long as you pass it it will be icing on the cake and might bump you up on the list since they know you won't end up failing it in April and delaying your start date.