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The last image in Phil's post deals with side taper - and yes I think the sides are similar to the top. But since they are adjacent to the bottom - and the ground, they are a bit different than the top. If they slope "too fast" (to use Phil's term) then they will pull air down from the top more than up from the bottom and change the air flow over the top, and possibly cause swirling at the back.
If the sides are not tapered enough, then the air lifts up to the top, causing a swirl in the other direction. (On the image of Phil's boattailed truck, you can see this slightly.)
I think the ideal is to have as even air pressure as possible all around the rear of the vehicle.
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