Mg, S, and Ca are considered secondary elements in plant nutrition, as compared with N, P, and K for macro elements and Fe, B, Zn, Cu, Mn, and Mo as micro elements. You want to be supplementing an appreciable amount of Mg and S, but you should be careful not to overdo it.
Sulfur certainly is helpful in flower (especially if you want to produce terpenoids and isoprenoids), it is a common deficiency that goes un-noticed because damage/burn only starts to show when S is VERY deficient.
Mg is very prone to deficiency, but the effects of it show more quickly (Mg is an essential component of many metalloenzymes as well as chlorophyll which is ubiquitous throughout leaf tissues--and is also a mobile nutrient by comparison to S).