I photograph my art and some surrounding area so I can correct for parallax if need be.
I never add generated borders, but I have a series of images of antique and other frames that I sometimes use, especially with digital art. see here and here
@anartist brings up a good point. When copying artwork for publication, it is best to over shoot with a loose crop. This allows for exact parallax correction later in PS. Much easier to do, as it is very difficult to get camera exactly centered, and most folks don’t have a completely distortion free lens made specifically for copy work.