And I am going to disagree with everyone and tell you to go ahead and do it. That is a strategy my husband and I use. However, and this is the part that makes it different. If someone takes the middle seat between us, my husband swaps with them. No one has *ever* turned down an aisle seat and said they wanted a middle one!!!
The odds are good that you may have to do this, but if there is an empty seat, it is more likely to be a middle one, especially if you pick a row near, but not at the back. At least twice, during the busy season, there was one empty seat in coach, and it was the one between us, and wasn't because the airline blocked it for us.
Good luck!
Jaelle