I have had some problems with getting good broadhead flght in the past, even though I was pretty well-turned. I came up with a theory, and so far it's proven true for me...
The issue is that when the arrow flexes as it comes out of the bow, and since the arrow isn't flying straight until the feathers straighten it out, that wing (broadhead) on the front of the arrow starts catching air and pulls the arrow off in a different direction. It makes sense that if that "wing" is positioned to cut through the air instead of planing against it, that maybe the broadhead can cut through the air and the arrow can straighten out before going off course. So, I took a few arrows and a torch out and shot a few. If the flight wasn't good, I heated the glue, twisted the insert in the shaft about 30°, and tried it again. Eventually I found a configuration that flew straight. I then twisted all my inserts to seat the broadhead at the same angle in relation to the cock feather, and I was good to go. I got great arrow flight out of those arrows. And since everyone has unique ticks and bad habits and arrow flight quirks, it makes sense that vertically-aligned broadheads work for some, horizontal for others, and various angles in-between for others still.
Lately, all my broadheads seem to fly well no matter how they are positioned, so I haven't had to bother with it. I am shooting big Zwickey's out of my 68# recurve this season, and they fly just fine.