Correct form and posture are essential when it comes to strength training. It not only helps prevent injury, but it's also necessary to ensure that you're working your muscles effectively. What you want to avoid doing is cranking your head to take a peek at the muscles being worked, because you can end up straining your neck or upper back.
If you have a tough time not glancing at your "guns" while doing bicep curls, I definitely advocate watching yourself in a mirror. That way you can keep your head in line with your spine, and also admire how ripped you look. Or you can videotape your entire strength-training session if you really want to study and perfect your form.