Zero!!
I’ve always paddled without feathering. A feathered paddle doesn’t make any sense to me whatsoever. Unless you have some asymmetry in your arms or shoulders that prevent you from paddling the same on both sides, feathering a paddle is not necessay. I’ve had people try to convince me that the “natural forward stroke automatically indexes the blade,” but that doesn’t make sense. A proper forward stroke is the same on both sides. With a feathered paddle, you have to index one blade by twisting the opposite wrist. This just brings on the possibility of an overuse injury.
The only possible argument for feathering is the headwind thing, but I agree with Eric_Nyre, if it helps with a headwind, it doesn’t help with a tail wind. I’ve paddled into some strong headwinds and never really felt like the wind was pushing my upper blade significantly. You can also use a lower angle stroke in a strong headwind.
I think Tiva’s remark is great: “I think lots of people feather only because they think it makes them look like a ‘real’ kayaker.” I think she nailed it!!!
Pedro Almeida
bent shafts
For those who like a bent shaft paddle, I might have a good solution to avoid confusion while rolling with a bent shaft paddle. Extend your paddle for rolling. Put one hand close to the center of the shaft and the other hand on the blade. As an added benefit, you’ll get more leverage/support while rolling.
Pedro Almeida
Left hand control
There is actually a reason even left-handed people might be better off with right-hand control. *What might that reason be?*
Anyway, it appears that lefties often use right hand control.
There is no requirement to have the control hand be your dominant hand.
Certainly, it's OK to use left-hand control, if that is what you prefer.
Racing
"A feathered paddle doesn’t make any sense to me whatsoever."
What do flatwater/open ocean racers use?
These people have a high level of motivation to discard techniques that “don’t make any sense”.
High angle
Of course, you can use an unfeathered paddle at a high angle.
Still, how many long distance paddlers use unfeathered paddles at a high angle?
Generally, good advice
It’s generally good advice not to keep mucking with the feathering angle. Keep it consistent unless you have a good reason to change it.