I'd like to know which types of objects need to have a separate OBJD for each tile, and which ones can get by with using the CRES to expand the footprint. No deco object tiles need their own OBJD. I believe bed/couch tiles all need their own OBJD.
As people have been adding more and more CC to their downloads folder, it's been discovered that there's some kind of OBJD limit, possibly controlled by the amount of RAM, that can cause Build/Buy crashes, so controlling the number of tiles in the CRES should be able to help people use more CC without hitting that limit.
Items that need to initialize a tile or some tiles differently to others need separate OBJDs for each tile, doors, windows, beds and hot-tubs for example.
Sometimes though it's not all that clear cut. Own-able vehicles always have a single GUID and extend their footprint whereas services vehicles never do, that's because service vehicles work out where to stop by calculating their length by the Multi-Tile sub index of their Lead tile.