Firstly, it probably depends on whether or not 
the studio released the cels with matching douga 
or not. If you collect only AIC cels, you won't be 
getting too many matching douga, so it doesn't 
matter. The point, I suppose, is that DBZ and 
other anime often will have matching douga and 
when they do, many collectors will writhe at the 
idea of separating them. OTOH - and this was 
pointed out in a previous thread about this, for 
very expensive cels like Lodoss and Ghibli, 
separate douga allows collectors with less money
to be able to afford a piece of the art. I 
personally prefer matching douga to stay with the 
cel. Genga, I think, may be considered separately 
from the cel - unlike douga which really is an 
exacting match. I wouldn't mind buying a genga 
of an anime that averages in the thousands per 
cel. *shrug* It's a gray area.