depends on how you have your rudders set up... pedals? twisty stick? keyboard or some other j/s input, i.e. rotary, or buttons to toggle input?
in any case:
using rudders in flight will help you roll faster than using ailerons alone.
when slow, using rudders to counter torque and/or help you roll will reduce the possibly of spinning or stalling out. this is helpful in slow speed scissors or in the vertical as you try for a rope (in which case it'll help you flip the nose down and around quicker).
using opposite rudder input while in a dive will help you reduce speed faster than just with trying to use throttle alone. it also helps you bleed speed when coming in hot for a landing.
using opposite rudder while making a roll/turn is called a sideslip. it drains off e and lets you turn a bit quicker. i.e. a "bat-turn" if you're familar with AW. it can also help you bleed speed to assist in trying to force an overshoot or conversely bleed speed to prevent your own overshoot.
some use rudder to make small adjustments in aiming. it really depends on your rudder setup and how sensitive it is.
mostly rudder use in combat comes with experience and becomes second nature. again a lot depends on what you use for input. rudder pedals are the most intuitive and proovide the finest control. twisty sticks can be hit or miss depending on how sensitive you have them set for. keyboard or other j/s input would probably be the most difficult to get used to and would probably result in less overall rudder usage.
