I think the 1 week before due date is a standard answer, based on the averages of how long babies remain.
When my niece was born 14 years ago, she had to meet certain criteria - she had to be at least 4 pounds, eating and breathing on her own while maintaining her body weight.
My son was born at the same hosipital, in the same NICU - he was born 1 week gestationally later than my niece (she was at 33 weeks and was in the NICU for 28 days, whereas my son was there just over one week) and had no breathing or temp issues, but had problems eating and struggled with jaundice.
There are no black and white answers - just experiences and averages based on those experiences.
Alice - survivor of PE twice and mama to Heinrich (Henry), May 2006, born at 34+2 at 4lbs 5 oz and Sofia, February 2009, born at 37+1 at 6lbs 5oz.
Very thankful for the PF - who I found after Henry was dealing with preemie issues.