I think it depends on if it's hobby or not and how much you think it will improve your maps and how much enjoyment you get out of it.
I used to sketch a little but I get much better results If I float around with the camera in my map thinking thinking and thinking. However 2 days ago I started sketching a little again, just so i'm not behind a monitor all day.
What I used to do is make a screenshot of a part of a map where I needed to continue. I would make the screenshot in such a way that on the right side of the image you could see for 30% or something a building I mapped. On the left side nothing except brushes for the floor. Then I would print it out like 10 times with 90% transparency so you hardly could see it. After that I would draw over the outlines of the building I already mapped and after that draw a building next to it from scratch (or a wall etc.). This worked really well for me cause you have guidance for scale and perspective and you can see how it adds to what you already mapped. I really recommend to try this out.
Anyway, If time is not an issue, go make concept art. Else I would look around for it.
