How about an interaction design like this:?

-press a button that says "Create ruler in 3DViewer" (or something)
-button becomes disabled while ruler is being created.
-see mouse modes change to "Place"
-click once in 3D viewer to position one endpoint
-move mouse to see rubberband line between endpoint1 and mouse pointer.
-click again to place second endpoint
-see mouse mode change to "ViewTransform" default.
-and see button become active again.

While endpoints are being actively placed, any leftbuttondown/release events outside of the 3D viewer are ignored.

Eventually, when 3D and Slice Viewer interaction are harmonized, the same interaction design should work in either viewer type -- whichever viewer type got the first click "grabs" the ruler, and second mouse click needs to be specified in either 3D viewer or Slice viewers.

I don't think there's a rubberband option in VTK5.6, nor a click to place for the ruler yet.
I reset the default position so it's inside the default annotation box, but it's not clear what we'll be able to do for the 3.6 release.

I think the new p-p-^m mode make this lower priority and I would vote for removing the slicer 3.6 tag unless you have a good idea what the behavior should be...

I think that I could use the fiducials with a special click to put down a ruler mode and a line between the fiducials to replicate the behaviour Wendy has outlined (which I like!), but at this point it would be a feature request level change. I'd rather have the time to do it correctly after the 3.6 rush, and/or work with Kilian and Yong to do it in the annotation module. Or work with Kitware to extend the VTK level functionality to make both the ruler and the angle widget be click to place.

If I can get the current bounds of the annotation box (changes are in Base/GUI/vtkSlicerViewerWidget.cxx, the BoxAxisBoundingBox) the next question would be, should the new ruler span the bounds as seen from the current camera position, or should it always be running along the same part of the box?

I've figured out how to get the ruler to appear diagonally across the bounding box now.
The bounding box isn't updating when new widgets are added because the ruler and angle nodes are transformable, not displayable. To be fixed in the miAnnotation module.

svn 12846 sets the default ruler position to span the annotation bounding box. Slicer4 annotation can work on the click to place behaviour.

Closing. Slicer3 is not being developed anymore.