Still killin time watching youtube videos waiting for the final piece of my 750. A guy in a video I saw tonight (with a shapeoko) said you can’t cut out square corners with a round bit which makes perfect sense and makes me feel stupid that I hadn’t thought of it. Then I started thinking but what if you did final detail with the smallest straight bit you can get which I assume is a 1/32 ? Would that small of a bit not make it almost impossible to tell it wasn’t perfectly square?
The outside square can be square but the inside cutout square is dependent on the radius of the bit. The eye is a pretty accurate measuring device, and you’ll still see rounded corners, but really it’s dependent on the size of the square in relation to that radius. Most woodworkers will knock sharp edges off their work with abrasive paper a bit, so that is a consideration too.
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A sharp chisel on inside corners works in a pinch. 
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On thinner pieces needle files are your friend.
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Got it. Outside squares yes inside squares no. Makes sense