Playing while you are sitting allows you do develop a bad habit of leaning the guitar so it is facing a bit upwards so you can see. The reason why it's a bad habit is it compromises your leverage fretting notes and grabbing chords. Christopher demonstrates this in one of his fundamentals lessons and talks about the "axis of leverage" or "axis of power" (I forget the exact term).

When you stand, you can't turn your guitar towards yourself, so you are forced into a more proper posture.

I started practicing standing up when I started playing in groups where I had to stand and play. It was a challenge at first but I adapted quickly. Now I will sit if I am going through a lesson, and I do that just because it's easier to control my PC that way. Otherwise, I'm always standing.