The problem with using RO water is useful minerals in the water are removed. So, in some cases, those minerals might need to be amended to the soil. It'll depend on what's already in the soil.
Water spots don't burn cannabis leaves. That's only true for plants that don't have the tiny hairs like cannabis leaves do. Without the hairs, the droplets spread out and become little magnifiers with the focal point on the leaves. That's how they burn. Cannabis leaves do have hairs, so the water droplets are round and there's no focal point on the leaves. So, no burn.