Hi Lady Rowena
to the above, i would add that if you are visiting a Bronze Age or (to a lesser extent) Iron Age city, polishing their decorations may be good for them, as they may not have been able to afford much in the way of cultural buildings yet.
But in higher cities than that, avoid decorations if you can, because they really don't make much difference to happiness levels.
However, if you are looking for blueprints for a Great Building, these are randomly issued when polishing, motivating or plundering buildings in someone else's city. There are no Stone Age GBs - so you will never get BPs from polishing Stone Age stuff - trees & obelisks & such. The BP gained is always from the same age as the building polished/motivated/plundered. If you are desperately hunting for a specific BP, it is often easier to locate a decoration of that era, especially if the player has progressed their city to a higher age.
Final point is that there are a lot of decorations in people's cities at the moment, because they were needed for the Easter Event (where we could hide the eggs). It would help visitors if people deleted a lot of these now that the event has ended!
