If you did awesome on step 1, DON'T take step 2 early - it can only hurt you b/c if you do worse it looks like you just got lucky the first time.
If, however, you got a mediocre score on step1 you should take it early. You can only improve, and if you don't it doesn't look any worse than having only one mediocre score. It's your chance to make an improvement on your application, and several 4th years have said it really does help.