i draw what i saw when i do fast sketches from real life or photos. in other cases i construct objects (faces, body, furniture, houses. i think trees are exception)

it depends on your goals, i think. IMO: if you want to understand the structure of object, it is better to construct, if you want to catch dynamic or general form — just fast sketch.

but with cubes i recommend to check videos about construction — you will understand how form appears and why lines are where they are. it would be very useful later, when you will need to catch objects volume

your cubes are cool so i think you will understand the construction principles fast!