I also think it's safer to back-tie until the handler can fully understand the concept on how to work the line during protection. That way neither the helper or the dog gets screwed over in the work because it's all in the control of the helper.
I had no idea what I was doing when I first started and no one ever explained to me how I (a 17 year old girl) was supposed to correctly post myself to hold back my 85 pound dog. Needless to say, I moved around quite a bit, because its awful difficult to not move when you're holding the leash with two hands completely extended
The first time I was ever told otherwise was at my last club's fall trial almost a year after I had started in Schutzhund. The judge was doing some helper work after the trial had ended and I brought my dog out. He told me to be a post or he'd whip me.
Guess who got whipped.
But seriously he was the first one to explain to me a better way to hold my leash, and had me wrap around and pretty much sit on it, and it made a world of difference (obviously). Three years later, I will approach any stranger working their dog who is holding the leash incorrectly At at club, trial, etc, and I'll show them how I was told to hold the leash. If I can hold back my dog, then they can definitely hold theirs. And all the time, I'm told that they were never given that advice before.
So just make sure you're not assuming that the handlers all know exactly what to do. It's scary enough already, being new to the sport, and a lot of people aren't being properly informed before they start.