Watch the Left-Hand Fingertips 1 online guitar lesson by Bruce Arnold from Guitar Physiology Survival Guide
Keeping the tips of your fingers parallel to the neck is critical for accuracy and agility. Reaching, or "leading" with a finger, or even reaching from the wrist can cause your hand to tense up and will cause the rest of your fingers to move up and away from the neck. Most students are so used to reaching that they don't even know they are doing it.
Remember you want to keep all fingers moving the same way no matter where they are. This will create the fastest interface between your mind and your muscles.