I have had my baby girls bag packed since about 28 weeks, but I haven't packed my bag yet. I figure I can do that when I either go into labor or before my scheduled csection at 39 weeks. I am still wearing my stretchy clothes, so if I pack them up I won't have them for the time being. I have my boys bag packed to go to grandma's as well just in case my daughter comes early. Both of my boys were early, so I wanted to have some stuff prepared ahead of time.
I don't really think there's a set answer for this. Only because everyone will have a different opinion on "when's best". I say, when you feel it's the best time... Some moms wait till they're a couple weeks from having baby, others, like myself plan to do it at 30-32 weeks. We truly never know when baby will decide they're ready to come... And I'd rather be ready than not.. As long as everything stays clean, and dust free, there's really no getting things ready to early in my opinion:-)