My horses are permanently on magnesium suppliments. Partly because I know the area is magnesium deficient, and partly because it keeps them a little calmer. I absolutely agree with using whatever takes the edge off, IF it is a suppliment. If it's a drug like ACE, don't even go there. Tranquillising your horse won't help its training any.
Taking the edge off with a calming suppliment, on the other hand, is brilliant, your horse is still himself but he's more receptive to what you're asking of him and much more willing to learn. I use rescue remedy ALL THE TIME on me, and on my riding horse. It's all natural, made from plant extracts, and you can't overdose on it. It doesn't make you drowsy at all. Just reduces the amount of stress and anxiety you feel. It doesn't work for everyone (or every animal) but it does work brilliantly for some. It's not a placebo, because if it was it wouldn't work on animals! (and it most certainly does, we have a dog that had serious issues when we got her and RR worked brilliantly)
I do think that it should be your decision what you do and don't give your horse, however you should research these things instead of just assuming they are bad!