It is recommended (but not necessary) to use Paper Masking on both sides in order protect both the mirror backing from being scratched and on top of or in place of the poly film to prevent it from adhering to your laser bed when cutting. Once again recommended but not necessary.
Always place mirrored acrylic with the reflective side down. Mirror your images before engraving or cutting.
Glowforge Users: Use PG Medium Red settings
Suggested settings are a starting point. Always test a small cut/engraving before running an entire project.