Every time there is a legal back row attack should that offical sign be given as the play continues?

Not every time. I would only do it if it's a close call between being legal and illegal. To do it every time would be overkill, and could take focus away from the next contact.

Agreed - if they're clearly behind the attack line no need to signal.

The one exception I make is with the libero. As it is illegal for them to attack a ball completely above the height of the net anywhere on the court, I will always use the safe signal when they legally attack the ball.

Had a real fun one in a varsity match last night: With her foot on the attack line, libero sets a ball above the height of the net that is then attacked by the outside hitter. I whistle & indicate BRA. The coach goes nuts and the captain complains to me it was a front row player attacked the ball. I told her that was true, but the set above the net by the libero while she was on or in front of the attack line made it an illegal BRA by rule. She just looked at me like I had two heads. If these people are going to use the libero, I really wish they would read the rules!