As a junior, I think its more important to play a lot of singles. That's the only way you're going to be noticed when it comes to college scholarships, IMO. Althought if you're just looking to win jr. touranments matches or maybe even go pro (crazy idea, huh?) then you may want to focus on being a doubles specialist. There are far fewer doubels specialist both on the pro tour and on the jr. circuit (as well as competing in the lower level jr. touraments). You'd do well to exceed in an area that is usually overlooked.
But if you're prepping yourself for a run at college, I'd say stay with singles.