I'm really horrible at explaining in words so I've drawn it for you:
It's a really subtle detail and I'm being picky so I apologize....

I've circled what I've talked about - then copied it and put the shading on the other side.



EDIT - the outline looks good :D

and yeah - I tried adding some of the shade 1 to it and you are right - it looks more plastic XD I say leave it without it :D I think that's why they give you the possibility of 5 OR 6 shades...because sometimes you dont' need the extra color