When I worked at a car dealership 5 years ago they had the computer monitor set off to the side of the desk with the keyboard directly in front of me and they said that was so I could see the customer and interact with them.
But I couldn't stand to use it that way with the computer monitor set off to the side and ask my boss if I could move the computer monitor back in front of me and then just move my chair over when speaking to the customer and the boss agreed.
I got more done and felt much better.
So in my opinion computer monitors should be directly in front of you and the computer monitor should be raised up high enough so that you're not having to look down at it.