The guy at my local shop, the way he explained it was that you dont need a helical but if your going to shoot fixed blade broadheads it best to just offset the fletching a couple degrees. Not alot but just enough to get it to go around once or twice. His thinking is this gives a little more drag but more importantly the broadhead will have less of a tendency to plane.

I cant say the guys an expert, although he certainly thinks he is, but I'll say I value his opinion. I'm thickheaded & usually dont listen right away if I think I already know what I'm doing. Anyhow, I cant say how many times I ended up listning to him after many frustrating hours trying to make myself right.

In my very limited testing of straight & offset it seems that broad heads are easier to control with offset fletching & the offset seems a bit more forgiving regardless of point type.
I had some arrows that came with a right helical 5" vane installed & they grouped about the same as the offset fletched arrows but seemed to be a bit noisier & I couldnt get them thru my rest reliably.

They now wear offset 4" feathers, against the local shops advice but I like feathers on my arrows.[8D]