If it's properly boattailed, the side area doesn't necessarily increase. Depends on speed vs windspeed and angle, but plan for the most common condition in your area.
Here's how I visualize it (with sort of off-the-cuff numbers): Look at the vehicle from the front, offset at, oh, 10 degrees. That is what the x-wind sees (pic 1):
Looks like proper tapering in plan view added exactly
ZERO to yawed "frontal" area and thus shouldn't adversely effect stability.
Perhaps this is 5 degrees yaw?
Perhaps this is why outside mirrors don't hurt drag much in the real world? (not adding much "frontal area" at all)