This is a complicated question.  A lot of very talented engineers use a compressor on their master bus.  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.  It’s very popular to compress the master bus.  HOWEVER.  It’s usually done pretty gently, to “glue” stuff together, and Eric “Mixerman” Safran likes to point out it prevents surprises when sending a mix off for mastering.  This is all true.  Where it becomes a problem is when it’s done to excess – 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 can’t be undone or adjusted by the masterer.