I'm not sure what the old handbook said, but I have a vivid memory of learning this lesson when I was a teenager. My YW leader had told us a story about how she or her husband had gotten sterilized, and then after a bit they ended up pregnant, and she felt like this baby was a special blessing that was meant to come to her family. My mom jumped on that and pointed out to me that the prophets had counseled against those sorts of procedures. I'm sure she didn't want me to get the impression that we can do whatever we want with our bodies and Heavenly Father's plan will come to pass no matter what.
That story, and your SIL's experience with her stake president, reminded me of Elder Holland's recent conference talk where he said God has imperfect mortals to work with and how frustrating it could be to Him. We don't always get the messages we should be getting from our church leaders because they are no more perfect than we are. I think if someone counsels against what is in the handbook, I would disregard that (since the handbook is technically coming from a higher source).
Anyway, timely topic for me, as I was studying on it last week. I'm 6.5 months pregnant and pretty sure this is the final baby. I don't think Heavenly Father has any more planned for us (at least it would be QUITE a while down the road, and then probably not through my own pregnancy), so I feel good about that situation. I'm just trying to decide what to do about it. I have mixed feelings.
On one hand is the open womb/closed womb idea, which I think takes a lot of faith and can be right for some people.
On the other hand is the idea of making a prayerful decision and then YOU and spouse exercise responsibility and take care of your birth control needs - which involves using the mortal agency that God has given us.
I think I lean towards the second philosophy, but I just really want to be sure that I'm doing the RIGHT thing, you know?
Anyway, Andrea, if birth control pills are not for you, have you decided on alternatives (if that's not too personal)? I have tried the pill which worked fairly well for me, I just have an environmental/health issue with them. I've also tried an IUD (non-hormone) which was only moderately successful as I had some side effects that I'd rather not deal with - but in terms of simplicity it was fabulous. AND I tried NFP which I think is totally ideal except that my silly body isn't regular so it's really hard to do (so for me, using this method involves a lot of faith in the open/closed womb idea). I've also had long periods of infertility (thus the reason I have some huge gaps between my children). That has been the most effective. ;-)
On another topic, a friend of mine shared an article about postpartum depression that I found pretty interesting.