Block out with simple shapes and just enough modeling to define things, very rough. This way you and push things around and focus on gameplay rough materials and some rough lighting. Then start refining things. Usually I'll go an object at a time and take it to completion.
Definitely grey-boxing a rough scene scale blockout is a must. It's also the time to test game play mechanics and how the player moves through the environment: door widths/jump distance/slope angle/ceiling height/etc.you can also take this time to set up texel density with a test texture which you can use later to author…