Without a doubt, yes. Mastering is always an important part of the creative process, and when your music is downloaded by a listener and loaded onto their MP3 player your songs will often be added to playlists with recordings from many other artists. Any deficiency in the Mastering on your songs will be readily apparent. Things like lack of level, EQ issues, noise, etc. can be quite noticeable when your songs are played alongside releases from major label artists.