Igg, The story I got on "slant cuts" is, way back when, before there were Piping Engineering groups and CFD. Some fitters and welders got together on a job site and decided that it would be best to put slant cuts on all the RV discharge piping. You see, the ellipse has more area than than the circle does, so it will make the valve relieve better. Or so the logic goes. I guess the rest is history and tradition and who the engineers, designers, fitters & welders trained under.
One thing is known, it increses the cost of the discharge pipe. BTW, don't forget the weep hole on the bottom side of the discharge pipe.