I need to sit down to sing because I play the guitar too

I just started taking these lessons. I notice that everybody recommends standing up for proper posture. In reality since I play the guitar too (Bossa Nova style), I need to be seated to do that. Why learning while standing when I play sitting down?

Standing is preferred for your best posture and support; so even if you eventually sit (for some performances), you'll have trained in the ideal position. When you're seated, sit on the edge of your chair so your feet are on the ground. Keep your spine long and be careful of the tendency to reach your chin forward or roll your shoulders forward. We want to be just as energized and tall in our seated posture as in our standing!