For what it's worth, I think there are a decent number of lagoon view rooms in both Conch Key and Sago Cay that do not have great MK views, so if you do request one of those at check in, make sure to note that it's really the MK you're interested in. (As I understand it, though, the lagoon rooms in these two buildings that do face the MK are among the best MK facing rooms -- they are nearer to the ends of the building. There just aren't as many, so lagoon view in these buildings won't guarantee you MK.) Virtually every lagoon view in Boca Chica, on the other hand, has an MK view.
If MK is important to you, the one you definitely don't want is Big Pine Key. Half the building is lagoon view, but faces the Poly. There are some rooms on the very end that do face the MK.