I usually read them when I finish.
I may not wait that long, but I tend to read them after I've delved in enough to know that the book is worth my efforts.
I kind of thought the same thing. But then the intro to The Fountainhead gave away how a relationship panned out, and I was a little bummed because I was curious. Not to say that's the most important point of the book, but I trust the author's judgement in leaving us to find out when she wanted us to.
The reason I usually read them after, is because often the intro talks about stuff that I have no context for until I read the book. This is strange to me, as introductions are supposed to lend context to what will then be read. Sometimes though, I do it your way. Especially if the introduction is considered notable.