Afaik, WW1 planes mostly used rudder for turning, I personnally always initiate turns with rudder and stabilize with ailerons. For good coordinated turns you always need some rudder (chasing the ball in later airplanes) to avoid a "slipping turn". You also need rudder with rotary engines to counter the gyroscopic effect which sends the nose up or down following which direction you are turning in.

http://www.youtube.com/watch?v=gniHY-IGxJ4

The tutorials by Requiem were made for Rise of Flight, but they are still generally useful.

* Of course you always have to use the rudder gently ! WOFF FMs tend to consider the rudder is the same as airbrakes !
( Which can sometimes be useful like when you come too high / too fast for a landing. Throwing your plane in a slipping descent is pretty effective in cutting down the speed or avoid taking too much speed. )

** Note on the DH2 vs Eindeckers : your main advantage is roll rate (because you have ailerons and they don't), you can change direction twice faster than the Einis. You could although possibly meet some Halberstadt DIIs in July/August.