Most medical image viewer software uses color coding to make it easier to distinguish slices in 3D views. It would not be hard to do (you can try by copy-pasting a few lines of code into the Python console).
Code snippet to show slice edges
for sliceNode in getNodesByClass("vtkMRMLSliceNode"):
sliceLogic = slicer.app.applicationLogic().GetSliceLogic(sliceNode)
displayNode = sliceLogic.GetSliceModelNode().GetDisplayNode()
displayNode.SetEdgeColor(sliceNode.GetLayoutColor())
displayNode.SetEdgeVisibility(True)
displayNode.SetLineWidth(3)
Should we do it, too?
My concern would be that it may make the views a bit too busy:
It might be particularly confusing when the displayed image is smaller than the frame (this happens most of the case):
It could be possible to set the boundary of the slice view to match the background volume boundaries, but it would be a bit more work and could be a bit confusing when we display other content, such as markups or segmentations (which are displayed over the entire slice, not over only the background volume).
Should we show slice view edges in 3D views?