I guess it depends how you define 'need' and how you define 'mastering'... but all the big software out there (Cubase, Sonar, Protools etc) is capable of exporting a stereo .wav file and using plug-ins to do eq, compression etc. so you can definitely finish a song all in the same software package...
Now to get into 'real' mastering (of which I know little to nothing about) that's a different set of requirements and most of it has to do with a proper room, monitors, maybe some specialized gear and most importantly... skill!