I think, for the reasons Jose points out, that you should err on the side of numbering more. In particular, an equation which you refer to more than a line or two after it appears should probably be numbered. I've also found this makes editing easier: If you insert a new paragraph, with several equations in it, you don't need to look for all of the references you've broken.
I agree with you, though, that it never would have occurred to me that "above" couldn't refer to a line on a previous page. Perhaps this is a difference between those of us who read on screens, with our scroll bars, and those who page through print outs?