This depends on what specialty you are pursuing, and what your Step 1 score is. I don't think most specialties care about seeing a Step 2 score when they interview you, so in that case you can take it late if you want. The surgical specialties prefer to see a Step 2 score.
If you did great on Step 1, it's probably wise to take Step 2 late (like, November or later), if all you wanna do is pass. If your Step 1 score is weak, getting a higher Step 2 looks really good, and of course you'd want the programs you're applying to to see that. The best thing is probably to talk to an advisor in your chosen field who can give you advice about when to take the exam based on your Step 1 score and how much residency programs in that specialty care about Step 2. Of course, if you're just starting 3rd year, you have a while before you need to do this... you probably don't know what specialty you're pursuing yet.
Hope this helps. There are many good threads about how to prepare for Step 2 in this forum.
