I have found that it depends upon the plane. Some of my warbirds will climb when flaps are deployed (Meister ME 109), while others will not (spitfires and hurricanes). So, my practice has been to not use any mix at first and see how the plane reacts and then make manual trim adjustments to compensate. Once I know how that flaps affect the plane, then I start with a little bit of mixing (a few percent, such as 3-5%) and slowly increase from there.