Depends on the composition and where the darks are and where the focus is. There is a lot going on in this composition and to keep from having it all dissolve into total chaos, what with all those different textures going on, I have been constantly overworking areas, going back to deepen shadows or overall tonality and making sure that all the various components of the overall composition look like they fit together. This is where my technique of layering helps a great deal by allowing me to work an area, move on to another area and yet another and then to return to the first area and make subtle changes with a layer of grey to deepen the tone or a gradation of dark to light to indicate roundness and depth and dimensionality. By not specifically laying in a dark area in its fullest sense right off, I can go back and go back and darken as I work, making sure that the levels of dark and shadow suit the overall composition and make things look 'real' and as if they were sitting within space, some near, some far.
As to a query the other day regarding just how I have indicated the frayed, weather-worn woven cloth draped at the top of the image, I will attempt a little tutorial this weekend, once this work is completed, and see if I can explain how it was done.