How important is it to be accurate with development times?
Sure it's flexible. The kicker is that if you make negs that print well at a medium grade, you have a lot more artistic freedom to BOTH lower and raise contrast, with the same neg. If your negs are higher contrast, or lower contrast, to begin with, you pretty much lose flexibility in one direction.
And, lets not forget that if you know what to expect in the darkroom, a lot of guesswork is taken out, which means less paper wasted, along with a lot more hair left on our scalps.
While its nice to have a safety buffer, it would be wasteful not to try to make negatives that are consistent, both density and contrast wise.