This is something I discovered to help me build my biceps. Many of you probably do this intuitively. It's really helped me optimize my workouts. The range of motion of biceps curls doesn't have to be up in your face. Keep your form as natural as possible. As long as you're keeping the weight out in front of you, controlling the eccentric (the lowering part of the movement) while maintaining good form, you'll see gains.
Make the biceps flexion your whole priority, which will generally mean bringing the dumbbells to about chest level and really feeling that bicep muscle!