If you theme them appropriately you get double the bonuses. I don't recall if theming occurs automatically.
In any case, it's also good to know exactly where are your yields coming from and keep things organized. I'd hate to have a bunch of culture and be left in the dark not being sure what was causing those values. The game already has enough of that as it is. You may also have "duplicates" and wish to trade for different great works so that you can get those theming bonuses.