First of all I've never made a leotard, so this is all guesswork theory, I don't think you will need any negative ease.
With a common old round neck tee shirt, the neck band needs to be stretched (negative ease) to make if lie flat and cling to the body. If there isn't enough negative ease it will stand out away from the body; but that is what you actually want for the back of the collar so that it stand away from the upper back and thus parallel with the back of the neck.
Also if the collar is open at the front and there is negative ease in the collar then it will try to relax/shrink and pull the body of the leotard with it.