One thing to keep in mind is that rifle bores changed from smooth to rifled to stabilize the bullet. I have used a straight clamp with some offset but changed to a helical clamp that was set up with just enough offset to make sure the feather is flat on the shaft. With rifles, a general rule is that within any given caliber, the longer (ie heavier) the bullet (and usually slower velocity) the faster the twist. You can over-stabilize the bullet. A "hard" helical will slow down the arrow, but it will be more stable. With a healical fletch, I have found that I can even shoot thru tall grass and the arrow still flies straight. I feel that using a helical clamp set with the minimum offset on the shaft gives a good flight with minimal velocity loss. The wind also doesn't seem to effect the arrow flight as much. Doc Nock is right in that the arrow must be tuned to the bow, Helical fletching should not be used just to help a poorly tuned arrow. Also, using a helical fletch, you can use a slightly lower fletch which means less deflection on the bow and less arrow noise.
Just my .02 worth.