Magnesium is very important for some reefers, and not so much for others.
My first tank was a 55 mixed reef. I didn't even know you could test for Mg. So, I would say you can get away without testing, as I never had any issues, and got pretty good growth from LPS/Zoas/Mushrooms.
I now have a 75G SPS dominant reef, and I can really see the effects of Mg being low. I start having trouble keeping my Alk up over 7, and start to see some nuisance algae that isn't normally there. I keep a very close eye on my Mg, Alk, and Calcium.
Kind of both ends of the spectrum.
So, to answer your question - It depends, IMO.