Sometimes my wooden buildings have wooden plank decks like the buildings in the following picture when they are part of a town:
When they are standing alone or are part of a lumberyard or something they do not have any planks like on this picture:
All my adobe buildings do not have any plank decks because they were built directly on the earth like on the next two pics:
All my houses are based on so called ABS-plates (german word for white plastic boards) approx. 1 - 2 mm thick (it is a matter of how big the building is which is standing on it) so that you can add details as crates, barrels, stones etc. to it. I think it is a kind of personal taste and there are not any rules for this decision ... hope this helps