In church I pray with my head bowed and my eyes close, because I was taught that way to pray when I was converted. I was taught it was respectful and so I stick to it.
Now, I'm not saying what I was taught was correct, or that I'm 'RIGHT' in what I do. But as it's not instructed in the Bible I am at liberty, and for me this shows respect for the Lord and for others in the congregation.
There is a practical plus-side...with my eyes shut if another in the congregation, using their own liberty, prays in a pose contrary to my own preference - well, I simply don't know about it!