What causes these round corners Nozzle diameter = .4
Extrusion multiplier = 1
Extrusion Width = .45    &lt;-- I feel like this could be reduced to fix it?

Layer Height = .3


I'm using Simplify3D.
If it's on each corner, and not the layer change corner, it's a combination of overshoot and the short "pause" of the printer when it changes direction.   

You can minimize it, but it won't go away 100% (due to overshoot from direction changes), usually models with a slight rounded corners work great.  If you, say, increase the jerk as an experiment and they get sharper (but you obviously have to deal with potential ringing artifacts) then it's due to that pause and acceleration after a direction change.

I would try lowering the temperature a bit to slow the flow/oozing of the nozzle and calibrate extrusion - just to keep the settings tight as possible to keep dimensional accuracy, but not under extrude obviously.   

If it's where a layer change occurs, tweak and play with retraction settings - such as coasting and extra distance on restart (you can put negative values here). Once those are set, as an advanced tweak, try to max the z speed... Obviously this involves motor current, testing for skipped steps etc... but this would make the layer change a bit quicker, to further reduce oozing, due to the delay in layer changes if retraction settings don't fix it. 

Usually, you can fix the corners with retraction settings but then it can mess up other parts of a model, since those settings are global... I wish slicers were more intelligent.