This is a complicated question.Â  A lot of very talented engineers use a compressor on their master bus.Â  There are a lot of software products that an in-the-box mixer can use to emulate the same high-end gear these engineers have.Â  Itâ€s very popular to compress the master bus.Â  HOWEVER.Â  Itâ€s usually done pretty gently, to â€œglueâ€ stuff together, and Eric â€œMixermanâ€ Safran likes to point out it prevents surprises when sending a mix off for mastering.Â  This is all true.Â  Where it becomes a problem is when itâ€s done to excess â€“ at some point 2bus compression is going to stop being a subtle glue and start becoming part of the finalizing process that the mastering engineer usually handles, and whatever you do as that process canâ€t be undone or adjusted by the masterer.