The bars shrink enough that shrink wrapping before 4-6 weeks leaves you with saggy wrap. Someone said they have to go back and reheat the shrink wrap even if they do it at 4 weeks because the bars will still shrink enough to make a difference over the upcoming weeks. I would only do it after a full cure.
I do mine at 5-6 weeks generally. However, I have found that even soap that has been cured a long time over 6 weeks will still shrink a bit. So, I just hit them with my heat gun to shrink a bit. It's quick and easy. I use the bands and it doesn't take long.
I do a full shrink wrap after the soap is cured -- no less than 4 weeks and often more like 6-8 weeks. The wrap continues to shrink over time, because soap continues to lose water even after the usual cure period, albeit much more slowly. But it's easy to reshrink if needed, as Shari explained.