I think there is some confusing of 2 different things in many of the preceding posts, which is misleading. During a turn there are 2 different events which would require the use of rudder, but only one of these would benefit from aileron - rudder mixing.

During the act of rolling into a turn and out of a turn, there may be yaw caused by the ailerons, there may not be, it depends on the design of the plane. Gliders are particularly prone to yaw by aileron because the long span gives a big leverage. Often the yaw is opposite to the roll, eg left roll right yaw, this is called adverse yaw. During this time while aileron is applied, rudder is required to counter the yaw and keep the plane straight on into the airflow. For this, aileron - rudder mixing is useful.

As soon as the plane has been rolled to the required bank and aileron is back to neutral, any Tx mixing is negated but there may still be a need for rudder. Banking a plane makes it start to move sideways, the fin's job is to act like the feathers on an arrow or dart and cause the plane to rotate around its yaw axis. Note that the wing changes the direction of travel, the fin/rudder change the direction of pointing. If the fin is doing a perfect job there is no need for rudder during the turn. Life isn't always perfect so depending upon conditions such as spiral airflow from the prop, power setting, airspeed etc, you may need a bit of rudder with the turn or a bit of rudder out of the turn to keep the plane pointing straight into its airflow. In full-size we just look at the slip ball, or the yaw string on the glider canopy. If the ailerons are at neutral because the plane is sitting steady in its bank but needs a touch of rudder to balance the turn, Tx mixing will be of no use. Of course some stable planes need a dab of aileron to hold the bank but here any rudder mixed in is countering the adverse yaw of the aileron and is not necessarily keeping the model in a balanced turn - after all it may need more, or possibly even opposite rudder to stay in balance.

So in summary, aileron - rudder mixing compensates for adverse yaw while the ailerons are being used to roll into and out of the turn, but does nothing to keep a balanced turn. That requires you to use your rudder thumb. Judging if you need rudder, and most models can get by without it, is a matter of judging if the model appears to be crabbing sideways into the turn in which case a small amount of into turn rudder will help.

Harry