I dont test for mag, if my cal is on and my alk is on I generally assume my mag is within range. done it this way for as long as I have been in the hobby.
I will test it when I have a problem with keeping the calcium or alkalinity in check, but generally, no, I do not test it often.
It is good to have the test kit on hand if you ever need it, though.
My tank has been set up for about a year and a half and for the first year I did not test the mag but I started doing it lately, when I first tested it was quite low so now I keep and eye on it.
i didn't for the 1st year my tank was set up. but after losing several sps, i tested, and the mg was at 1000. so i've been dosing and testing to make sure that everything is in check.
I do test magnesium about twice a month. It is always around 1300, but I get bored and test it anyway since I paid for the salifert test. I might as well use it.
The only other tests I do are alk., calcium, and salinity ....