i did hear 2 blade heads should be lined with the string...or maybe it was perpindicular to the string? i dont remember...
I quit trying to figure it out years ago and wound up just setting them at 45 degrees. That way I'm at least half right. [8D]
Okay, so I lie.
Seriously, I have tried that and it works just as well as any other way. With the string, horizontal to the string, any angle in between, it doesn't matter. Let personal preference be your guide.
The only thing that truly matters is getting the heads aligned with the shaft so they spin true.