Look straight down as you swim
Let water cover the back of your head
Get your head down and out of the way of the stroke
Many swimmers hold their head too high in the water.
Lifting your head to look forward drops your hips and legs, which slows hip rotation and increases resistance along your whole body (our goal is to decrease resistance, not increase it). Better to relax your neck, let your head fall deeper in the water, let water cover the back of your head and look straight down as you swim.
It costs energy, more than you might think, to hold your head up lap after lap. With a neutral head, tension in your neck and shoulders will vanish, you'll balance better, and your hips will rotate faster.
A neutral head position is desirable in all the strokes.
Head position is, of course, closely related to breathing.
Head position affects balance, breathing, recovery, and hip rotation - pretty much everything.