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. Of course I also use the split view. 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, 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?
This is something few customers have asked before, Check out this discussion Lightroom: Eliminate the "master/copy" relationship for virtual copies | Photoshop Family Customer Community and add your vote as well.
Let us know if that helps.