Mg is very important, however depending on what you have in your tank and what additives you use you may never have to dose it. It is included in balanced salt mixes and in some cases with regular water changes is all you will ever need. However, it is very closely related to maintaining proper pH, Alk and Calcium. If you Mg is off, you may find it hard to keep a proper buffer going which can have adverse effects. The best idea is to get yourself a test kit and keep an eye on it, if you do regular water changes and your Mg always seems to test good, you may never have an issue with it. If not, you may need to dose it occasionally. If you drip or top off with Kalkwasser you are almost assured you will need to dose Mg.
