My LWA rolls away from sideslip. This may be a true, albeit sanitized, statement. HWA may have some geometric dihedral as well, although usually not as much as their LWA counterparts. Remember, it's the overall effect that the pilot experiences. He cares which way the plane rolls when in a sideslip, not the individual contributions of geometric dihedral, wing-fuselage interface, vertical tail effect, propwash, etc.
Interference drag arises whenever two or more objects are close to each other in an airflow.
The total drag can be more or less than the sum of the drag of the individual objects. The less-drag case was used by a close, personal friend to get better gas mileage driving to Florida. By remaining one car length behind 18 wheelers, the drag on his Vega may have been reduced sufficiently to raise the miles per gallon to what he would have gotten had all four cylinders been functioning. Regarding airplanes, wherever objects meet, there's likely to be a drag increase. Such is the case at the wing - fuselage junction even in balanced flight.