With the Deflect Arrows perk, you receive no damage as long as the arrows hit the shield. Because Ysgrammor's Shield is the largest (I assume) shield in the game, this perk is best used with that shield.

Well, there are no kite shields in the game unless you count the Imperial Shields, so your legs and feet are largely unprotected. If I was to crouch while using my shield, would my shield be able to deflect arrows that hit me below the waist?

Also, while in third-person, your character only 'blocks' in a circle: front, sides, and back. You cannot block upwards, nor downwards, or anything like that. However, if you go into first-person, your character will block in whatever direction you're looking - including up. So technically, you could hold your shield up above your head while in first-person, which you would not be able to do in third-person.

So my other questions are, does the Deflect Arrows perk work better in first-person? If I 'look up' while blocking, could I technically deflect arrows from someone up above me?

I know that with the perk that allows you to resist magic when raising your shield, the shield can be anywhere and you will still resist the magic.

It's probably the same with the Deflect Arrows perk but I'm not certain.