School me on unity gain...

Here's one for you. I made a rough mix of a track a month back, and just to get something in the ballpark I tossed a compressor on the master to boost the overall mix a touch (I never put anything on a master fader when mixing usually). So I came a month later and started to actually mix this track. I worked on it all day today, thinking that my master fader was set at Unity gain (zero). Then after finishing I noticed that I had that stupid plug in on the master fader. Now I took it off and my track peaks out at -1.4 I'd like to get it back up to zero. So what should stop me from just turning the master fader knob up to +1.3? I've always heard that you shouldn't do this, but I've never gotten an explanation why. I've moved the fader knob myself before on my rough mixes and I've never noticed any difference. Any knowledge on unity gain and as to why I should or shouldln't touch the fader knob would be appreciated.

Thanks for you reply. As far as dithering goes, I initially was told to put a dither plug in on the master fader and bypass it and record to 24 bits, and then to activate the dither and then choose bounce. But in a previous mix, I have forgotten to put any dither plug in before recording my mix. The mix was still in 24 bits. I think put the dither on the stereo channel it was mixed to and bounced it down. This achieves the same thing right? Or should I do the way I was initially shown? Thanks again.

hi, school yourself: regs