I try not to take extra through supplementation (but end up getting some in my general multi anyhow). Generally, I manage to get enough through dietary means alone, including re-mineralized water.

I find I take my supplements only when I cannot get what I need through food sources. You should try working more magnesium-rich foods (or water) into your low-carb plan; The body is fickle and you may benefit more from eating the right things rather than measuring the right amount of pills. Just a thought.