I don't know if it's typical of CG sketches, but 
it's pretty common to find colored shading and 
even comments on the back traditional sketches 
used to make cels.  I always assumed that since 
the cels were painted from the back (and so in 
mirror image) that the dougas were put on a light 
box and viewed from behind, and that the cel 
painter (or his supervisor) normally wrote or 
sketched on the back because that was what s/he 
was looking at.  

Don't know why this should be the case for CG 
work, since I assume the images are made straight-
up, not "from the back."  But maybe it's still 
easier to represent certain kinds of shading on 
the back and then view the end result on a light 
box.  Interesting question!