When I do soft proofing, I always create a proof copy, and I always use the Before/After button on the Develop toolbar (the button that looks like YY) to display the master photo side-by-side with the proof copy. During soft proofing, I only make changes to the proof copy.
My question concerns the Before menu that appears on the toolbar when a Before/After view is active. Since I want to compare the proof copy with the master photo, I select Master Photo from the Before menu.
Is it really necessary to do this? I haven't been able to find any difference between using Master Photo, Before State, and Current State. They all display the same version of the image in the left pane. Perhaps there are use cases where it make a difference which one I use.
Am I doing the right thing in selecting Master Photo? Is there a danger in choosing Before State or Current State?
FYI: LR CC 2015.10.1 (latest version)
What you are doing is correct. If you have made changes to the softproof copy of the photo and then you select Current State, the left side version will reflect the changes you made to the softproof version. If you wish to always softproof relative to the Master copy. continue doing what you are doing: work in the virtual copy and select Master Photo in the toolbar. For a more complete explanation, which takes some paragraphs, see page 492 of Martin Evening's Lightroom book.
Typically, I don't work this way for two reasons:
(1) Lightroom always preserves the original imported image in its raw state along with all the history of whatever I did to it, so nothing gets lost or permanently altered (the beauty of working in LR).
(2) I know what paper and printer I shall use for printing the photo and once I've printed it, I seldom go back to it. In these conditions, it is reasonable not to bother making a virtual copy for soft-proofing, but simply to enable softproof on the original master photo and adjust it till I am satisfied with the visual appearance of what will be printed. The exceptions are for radical changes of state that I wish to preserve separately, such a B&W conversion, or radical kinds of edits to make a sea-change of image appearance that I wish to preserve as separate interpretations for artistic purposes.
Now, having created a softproof on the master file and printed it, if I wish to change papers for another kind of print, say a major change from a Luster to a Matte paper, I would then create a virtual copy of the photo in its current edited state, and on that virtual copy make the changes needed to suit the different paper, which switching from Luster to Matte would be quite substantial. So I then have two valid renditions of the photo for the different papers.
They key thing about my workflow is that I'm not initially wedded to the non-softproofed version of the photo as a basis of comparison for soft-proofing. I do use it initially to see the full dynamic range and shadow detail my display allows, and that is useful guidance to start with, but from thence forward, all that matters is what I can output, therefore I optimize the softproof appearance of what will print within its own constraints (based on the histogram but more importantly visual perception on a well calibrated/profiled display).