So you've been told not to bother scanning ceilings with Dot3D. But why?
Essentially it's more just: If you don't particularly need them, don't go out of your way to add them. If you do need them, then of course capture them. Here are the reasons why it's discouraged if not needed:
1. Ceilings are typically flat and empty, and often extra close (or alternatively extra far) from the camera, all of which can result in Dot3D not tracking as well in those scenarios.
2. Time spent pointing at the ceiling is time spent essentially "breaking up" the otherwise continuous trajectory of your scan through other typically more important areas of the scene (floors/walls/furniture). This can sometimes add extra challenges/complexity to optimization. This is not inherently a problem, but can increase the risk of something going wrong.
3. It adds to the capture time.
4. It adds to the optimization time.
5. It adds to the file size.
So in short, ceilings are not recommended if you don't need them, but if you do, it's recommended that you take extra care to capture them from a bit further distance / with other areas of the scene also in the frame whenever possible.