Gravity doesn’t care what order you’d prefer to clean in. Dust, cobwebs, and grime sitting on your ceiling and fans will inevitably drift downward once disturbed, settling onto whatever’s below: walls, furniture, and eventually the floor. Clean top to bottom, and that falling dust lands on surfaces you haven’t gotten to yet, so you catch it all at once when you finally sweep and mop. Clean bottom to top, or skip straight to the floor, and you’ll be redoing that last step all over again.