calmag is for returning lost nutrients to distilled/RO/deionized/rain water.
If you need it at all, you need it through the whole grow.
If you're just using plain tap water, you may not need it, but something like silica blast that has Ca, Mg, and Silicates in it is still very helpful to strengthen the plants against heat, drought, and disease.
Hey there's plenty of other helpful folks on here too... don't worry!!! They are much sweeter than I am anyway... =p