Your neck can't be cranked backwards when you squat, nor can you be looking straight down. The ideal position is with the head pushed slightly back and the chin slightly tucked. Slightly. It's not excessive.
Sure, many coaches will tell you to look up. Why? Because they don't squat heavy and don't expect you to either. Know what's worse than herniating a disc in your lower back? Blowing up the tiny discs in your neck that you use literally to eat, speak, and live. It's a bitch.
You can look up with your eyeballs without cranking your neck backwards. Where you look isn't that important. Some guys look up, some look straight ahead, and some look down. What is important is to keep a neutral cervical spine regardless of where your eyes go.