Autodesks repsonse to this problem since 2009 implementation bug reports....
Aligning dimensions on angled walls
You want to place a dimension on an angled wall that is part of a larger wall system, but you cannot configure the dimension to run parallel to the angled wall.
If the angled wall is attached on both ends, then the dimension will not be aligned properly.
However, if you remove one of the walls adjacent to the angled wall, and then select the angled wall, you will create a temporary dimension that is aligned with the angled wall. When you make the dimension permanent, it will stay angled. You can then replace the adjacent wall that you removed. Follow these steps:
Select one of the walls adjacent to the angled wall.
Press Ctrl+X to remove the adjacent wall.
Select the angled wall to display the temporary dimension.
Click the Dimension symbol displayed to make the temporary dimension permanent.
On the Edit menu, click Paste Aligned > Same Place. This will replace the wall that you removed.