I guess I’ve not seen a need to do that when I’m modeling pieces of furniture or whatever. I know what the dimensions are as I am modeling. I’d only use something as a size reference when showing the model to someone else and if the scale of the piece isn’t obvious. It depends on what the piece is. It also helps to make sure textures are at the proper size when you add things like wood grain or other textures that might give clues as to the size.
As I wrote before, you can download models from the 3D Warehouse if you want other objects for reference. Or maybe you could use the default figure included in some of the templates included with SketchUp.
Something like this key probably doesn’t need any sort of scale reference but the figuring out the size of this clock in this image is probably helped by the furniture pieces.