If you have a Step 1 score of 240+, there is absolutely no reason to take Step 2 prior to January your fourth year. If you do really well on it, it doesn't matter because your step one score if already good and no one will really care. If by some reason you have a bad day and do not do well on it, people begin to wonder if your Step 1 score was an abberation. Don't get cocky. Don't take Step 2 early if you are in this situation, the risks infinitely outweigh the rewards.
If you have a Step 1 score between 230-240 there is a gray area. If you aspire to top notch programs and think you can do better on Step 2, then take it. However, you should have no problem matching to some solid programs with scores in this range.
If your Step 1 is below 230, then I would strongly consider taking Step 2 prior to interviews. It can be most beneficial in that situation, the rewards outweigh the risks.