Is it good practice to smooth a carved background with sandpaper or files instead of a chisel?
Not generally. New carvers are often tempted to use sandpaper or files to compensate for difficulty controlling a chisel, but it is better to learn to slice the wood off cleanly and read the surface by eye. A file can be reasonable in big open areas, like along the wrist, but it is not practical or a good habit in tighter carved areas.