It is important to have a slight lean forward in Niko Ashi Dachi as the stance is often used to push off, kick or strike an opponent.
If the back is vertical it has no forward power. You can test this by pushing against an opponent (try the Mawashi Uke block with the double armed push) and see if you have any forward power with a vertical back. You will soon notice that you need a slight lean forward to execute any sort of power.
Most of the weight should be on the back foot with the front foot used for stability.
The elbows should be tucked in and the hands on the bodies center line, the heels should also be on the bodies center line.
The groin should be hidden and protected by the legs and the hips.