Good voltage and resistor value for string of LEDs I building multiple parallel strings of 4 LEDs in series.

Since the four LEDs in series have a combined forward voltage of 12.56V at 20mA, and it's powered by 2 AA batteries, I have added a voltage booster.

Now my question is, since I can both choose the output voltage of the boost converter, and the resistor added to each string of 4 LEDs, what would be a reasonable value for both?

I could set the voltage to exactly 12.56V and not use a resistor, but that would probably lead to uneven current spread between the sets of 4 LEDs, as their forward voltage can differ slightly due to manufacturing differences.

On the other hand, I don't want to set the voltage too high, and wast a lot of power. 

So what would be an acceptable voltage drop over a current limiting resistor?
Since the relation between voltage and power is quadratic, the obvious answer is "as little as possible".

Allowing for 3% variance in VF gives us just under 13V, so that is a reasonable starting point. Solving for R gives 22ohms, a E24 value. This will result in a 8.8mW loss, which even a 0201 resistor can handle.