/3/ - 3DCG

>>976467
Both can be fine, depending on the context. If it's a model that doesn't move, and you're not looking to subsurf (or having a continuous subsurf), then having the flat faces is fine. If you're looking for a continuous mesh, then trying to make things flow cleanly will help later on if you decide to subsurf or bevel.
Hopefully the pic helps out a bit to show the differences and similarities of the approaches (assuming the object on the left of your image is 2 non-continuous meshes stacked). The biggest difference is how the models interact at the junction point. The continuous mesh provides a smooth transition, while the separated one does not. This can be preferable depending on your use case.