We prefer not too, but sometimes the client has a poorly mastered version of a song, doesn't have access to the raw mix, and wants it to sound better.  We can make the song sound "better" but if it's very heavily compressed to the point of distortion and highly enhanced, the finished product isn't going to sound very good.

On the other hand, many times the results are surprisingly very good.  You'll never know until you let us try!