babies (and moms) do best when they are delivered close to the 39/40 wk mark. which is not to say there aren't lots of non-complicated births before or after, but 39/40 is the best.
My mom went several weeks past her due date for my youngest brother and ended up needing a c-section, which she hadn't needed for 3 previous pregnancies. Might have happened anyway but maybe she could have avoided it if she delivered on time.
Something to aim for but obviously there are special circumstances (as Sarah pointed out).