Last year I purchased 2 standing mirrors, one that stands astride my monitor viewing area, but is ahead in the distance about 7 feet, and one that is directly lateral to me, that lets me check my posture every so often. I think the lack of good data for "how I'm sitting" can result in unsavory posture choices throughout the day, so by adding data or visibility to the equation, I am now able to do something about it. Sure, you can do it with one mirror, but something about having two distinct angles gives extra nuance for tilt and lean.