I almost always watch it after release date. I don't have a good reason as to why, but I usually find it interesting so. At least, for Haruhi I thought it worked out fine, even though it started out a bit confusing. It also works well for anime that aren't going anywhere with the story. The extra episodes are typically filler material, anyway.
Spice and Wolf is the only series that I can think of having done an exception. It was mainly because the story was put together chronologically, and watching the extra episode at the end wouldn't be very logical. The episode was set up to take place in the middle of the story.