If the plane has SPECIAL wings like a Delta, tailerons or elevons. For 98% of the average planes out there, you won't need them. Mixes like flaperons are common but should also be used carefully as they can be over-done. Sometimes a little elevator can be mixed in when flaps are used but this can be done anytime. Multiple aileron servos or dual elevator servos can require a mix, depending again on the set-up.