When the dentition is stable, many of the occlusal factors are well defined and they do not require any alterations. However, when faced with a heavily restored dentition with unstable occlusion, you may notice the occlusal plane becomes quite irregular. Combined with the need to replace numerous existing restorations, you may have to consider improving the occlusal plane for a better prosthetic outcome.
Personally, I like to use the maxillary landmark such as the Camper’s plane (ala-tragus line and the interpupillary line as my reference. This approach is esthetically driven and minimizes the risk of canted maxillary teeth.