Remember that Verin was bound by the Oath Rod to serve the DO until the hour of her death. The letters that she gave to everyone contain information that would violate this oath. Thus, in order to give them to Mat, etc., she needed to believe that they would not open them until the hour of her death (which was about 10 days away, I believe). Now that she's dead, I don't think it makes any difference when they open the letter.

Someone let me know if there's another reason for her time-delayed letters. It's possible that she had to ward them with a burning trap until her death to ensure she wasn't helping them, and she simply doesn't want them to lose the contents.