Some people say that magnesium is important, personally I haven't found this to be true. By the way, 1200ppm magnesium is about the proper range that most people find suitable in their aquariums.
If you have a high biological need for magnesium, such as many sps corals, a ton of coralline, then it is probably good to test for it etc.
In a coralline algae cookbook that I read awhile back the author described how to dose kalkwasser (which I do, and have had wonderful results) and to also dose magnesium, which makes coralline algae explode under certain spectrums of light (I think 14-20k MH's)
Good luck!