Have you tried section tools? In V6 its on the menu in V5 you might have to download it.
I highly recommend this and panel tools from Rajaa.
Also you can assign an mplane to the clipping plane and this should allow you to get accurate sections.
In the attached file I first created the rectangle at the odd angle to snap the clipping plane to then I attach the mplane to the clipping plane now you can move rotate the clipping plane and your cplane/mplane will auto update. Also if you set the mplane option to universal you can get all standard ortho views to update as well. In the file turn gumball on and you can easily adjust the mplane and see the section update. You might have to use section command and or make sure project is on when you work at the section or use section tools in addition.
AutoSectionCPlane.3dm (258.3 KB)