If you are laying down the first layer of this scene, like the green upside-down "L" and the beige one, is it necessary to keep the shapes clearly divided? Would it be OK to let them run together? How can you tell?
The answer is probably more in your gut than in the photo, since the photo displays a very distinct edge. It's really a matter of what you're comfortable with. You can do whatever you want.
Ask what is important to you. Light? Space? If you do decide to give control back to the paint, will the important things still be possible to realize in later layers?
How about the sky? If the green and beige flow into the blue, will those darks on top of the building separate them again?
These are the kinds of questions that can help you decide where you need to be careful and where you can be casual.
Here are a couple more images that offer opportunities to play fast and loose in the early stages. I find that "What the Hell!" is a pretty good guideline. It's only paint, after all.