I think you want to be accurate in case anyone uses your tree for research in the years to come. So I don't recommend just putting the middle name in the name field. You have great control over how the names appear by clicking on the "Show Options" link at the bottom of the screen.
My solution to exactly this problem is to enter the names in the name field like this: first middle "middle". But I started doing that before there was an explicit nickname field. As it turns out, this is exactly how the new nickname field is displayed. So a cleaner solution for you might be to put first and middle in the name field and middle in the nickname field. Then you can decide which to display.