As long as your ass is still touching the bench, and pinching shoulder blades is fine. Your back shouldn't be flat, you should maintain its natural arch.
If your not feeling any strain on your neck it probably is not a concern. It would DQ you in a meet but I doubt it would really help you lift any more weight.
i've seen guys turn their head, that is bad technique and needs to be corrected. keep your head planted, do it right.
i never correct guys at the gym unless they ask for my input, if they really cared about things they would watch the guys who know what they are doing or youtube some vids. if someone continues using poor form even with all the tools we have access to then they just don't care about anyway. when i started lifting back in the early 90's all i had was old books from the library.. today we have so much more tools at our disposal.