like someone mentioned, the slant is a design to allow maximum efficiency flow of the exhaust through the reduction of bends in the piping....
for example, take a look at this JIC exhaust for the STi.

there are very few bends and the bends that are there are at the lowest angle possible...
whereas this Borla exhaust that uses a standard muffler has 2 higher angled bends right before the muffler...
in many cases, those that have attempted to slant their exhausts have done it simply for the looks and actually end up putting a large angled bend right before the muffler in order to get the slant look....
for those that are familiar with the JUN 4th Gen, they have prolly seen this picture showing their custom catback exhaust with a similar design....this also makes it superior in terms of flow...