My last trip in August, we booked our ME but did not give flight information until 2 days before we left. It wasn't an issue. We had received our packet because we booked, but like I said, we had not confirmed flight information.
The above poster was right, just book it, if something happens, you have a space either way.
On the same note, my sister had to leave unexpectedly two days earlier than our planned trip home. I stayed behind at Disney. We were in the middle of Summer Nighttastic Fireworks none the less so needless to say we didn't get home, haha, to the hotel until around midnight. I went up to the front desk and was able to change her flight information and ME picked her up bright and early the next morning no problem.
Now, my only issues I had with ME that trip were these two things. When I went to change hers, they had our regular reservations at the hotel concierge, but when they called ME, there as no record of us having reservations on our original leave date. I was sad she left but at the same time it was good because I would have been waiting for the ME and not known that they had lost my reservation [I did have a voucher they had sent to me, but just possible could have been a headache]. Also, the day before I left, I never got a ME packet on my door. It wasn't an issue because I had a printout from when I arranged hers as well as a voucher at that point, but it somewhat stressed me since I saw them hanging on every other door but not mine [not that I wanted to leave, but you know].
So that being said, book the ME, if you have to stay behind or go later, you are covered, as well as if you were to happen to leave early, you'd be fine.
Good luck!