Several northern-state DNR's specifically advise fertilizing, including a regional Aquaculture resource center for several states including Montana, Washington, South Dakota, etc. I think it depends more on the individual pond's natural fertility than a blanket rule. If you have a pond that is naturally very fertile, fertilizing could be a very bad thing in the North; if on the other hand you have a pond with not much in the way of plankton, it doesn't make a lot of sense to me to deprive your fish because there are other ponds hundreds of miles away (or five miles away) at your same latitude that have more plankton naturally and are thus not good candidates for fertilization.

Just my $.02.