Unfortunately, no one can answer this question correctly for you. You can't predetermine if a plant will need calmag or not. If you are using a buffered coco, there should be no need to add any extra calcium, even if it's from a water source. That's why most coco growers use RO water. Calcium is a secondary nutrient, which means it's not needed in high amounts. Plus calcium is in almost every nutrient line. If your tap water is hard, it's hard because it's high Calcium. That's what hard water is.. It's water high in calcium and magnesium. That's what causes that white build up on showerheads and shower doors/curtains... That's dried up calcium and magnesium. If you go with a pre buffered coco, use RO.. If you get non buffered coco, the tap water will work ok for a while, or until too much calcium/magnesium has built up in the coco.
There are a bunch of variables at play. The best bet is to learn how calcium and magnesium work, what they do for the plant etc. Same with coco. Read up on cec exchange and buffered vs non buffered coco. It will help in the long run, trust me.