you should NEVER take your seatbelt off in-flight. Thankfully there aren't more injuries due to turbulence. I think if I wear my seatbelt in a car for 6hrs, what's the difference wearing a seatbelt in a plane for 6hrs?
You just never know when or where clear air turbulence will occur. Just keep in low and tight to be safe at all times...even if the sign is off.