Mastering is a process that includes limiting and compressing your mix to optimize track loudness while retaining the dynamic qualities of your track.

To get the best results from any mastering experience, you’ll want to minimize your use of these effects on your stereo master channel.

Also keep in mind that headroom isn’t just affected by your master fader and that you’ll need to be considerate of when you’re using compression and limiting effects across your individual channels 

Regardless of whether you’re using these effects, you’ll want to give the mastering tool (or a real-life mastering engineer for that matter) enough headroom to do its thing by making sure your output gain is low and the meter on your master bus peaks at 3-6 dB below zero.