I think knowing something about the heifer's and bull's pedigree and/or history is a big factor in deciding whether to intervene or not.  If the heifer's dam had no problems calving and if the bull has a history of siring calving ease calves/good CED epd, I'd let her go do her job.  It seems to me that we tend to at times intervene when we should let the heifer do her job.

On the other hand, if either the heifer or bull have calving problems in their pedigree, that is a different situation and we need to keep an eye on her. I remember the first couple of years I had cattle I was as nervous as a new dad in the maternity ward at calving time.  If we do our job in selecting calving ease cattle, especially heifers, there should rarely be a need for us to intervene in calving at all. jmho.