When shooting greenscreen I also like to give a little hairlight from above & back
My company requests that this is not done for greenscreen shoots unless it is totally justified by the the location lighting. Anything that breaks the true lighting of the BG is a problem. Anything that might enhance the edge is also a problem as we are trying to blend the actors into the background.
The worst enemy of greenscreen shooting is not an uneven background and I don't think the half stop difference are even remotely in the range of one thing: spill.
Blocking out all green (including from below!!!!!!!!!!
) except for the green which makes for the siloutte of the actor is going to help.
Number two thing I worry about is motion and relative motion in the scene because motion blur is really hard - even harder when people are moving at different speeds though. It means you have to do a split roto and handle them separately. Usually for feature effects we are doing about 8 keys or so every shot (dividing the character into head, arms, body and then other whatever the shot needs).
Number three thing... we sometimes have problems when green gets into dark areas.... but that would be a part of the spill issue.