I'm of two minds on Rousillon.
First off, on the map the province is MUCH bigger than Rousillon ever was in real life. Note that the province is PLAINS for crying in the soup- not what Rousillon is in real life. All the extra territory would start French. To me the most accurate solution would be to add Rousillon to French starting territories, leave it French culture, and let Aragon keep the shield there (not sure whether Spain would get one).
That said I think the reason Paradox gave it to Aragon was to promote conflict in the area, which is very appropriate, and to maek France work to get it's homeland. These seem to me to be appropriate goals. Changing the culture of Rousillin to Iberian is historically unwarranted (to me) but it is not unreasonable. The net effect of the change would be to change the balance between Spain and France in Spain's favour, which I like.
So I don;t think it makes the game more accurate, but I do think it makes the game better. I have to abstain on this one.