Well, some people think that almost any reason is ok for using NFP, which is not correct. The reason must be unselfish, and it must be just/serious etc enough for a couple to decide that having a child at the moment would not be a good idea. Remember, just because one is not using NFP to avoid does not mean you will have a child every time you are fertile.
Each couple is different and in a unique situation at any given time, so its hard to give concrete examples (except for the obvious life death situations). Each couple must pray carefully and discern what God is asking of them. I agree, its not a thing that you go and get permission from a priest etc (heck I know of plenty of people that have gotten advice from a priest to use ABC or much worse, in one case, to have an abortion). This does not mean you should not consult a priest (the examples I gave you were of priests that do not have things right, but there are plenty of wonderful priests out there), in fact one can consult one’s spiritual director or any priest you feel will give you spiritually/theologically sound advice. In the end, its up to the couple, since they will have to face God later with their decision.
I like the way someone else put it in another thread, NFP is not a substitute for ABC, it is a substitute for having to abstain completely (I add: while a certain condition exists, for any amount of time, that can make having a child at the moment be a bad idea). There may be a certain period of time that a couple thinks they may need to avoid conception, but they are responsible to continually evaluate the situation to make sure the condition is there and that it is still serious/just. NFP to space children is a tool that has been given to us for when we face situations in which we carefully have decided that having a child at the moment is not a good idea, which is different then using it until the ideal moment to have a child comes.
From the Catechism:
For **just ** reasons, spouses may wish to space the births of their children. It is their duty to make certain that their desire is not motivated by selfishness but is in conformity with the generosity appropriate to responsible parenthood.
BTW, for reading on the subject I would suggest Kimberly Hahn’s “Life Giving Love” book, Humanae Vitae, and anything by JPII on this subject