it prob wouldnt hurt, though i'd only add a small amount (i.e. 1 teaspoon per gallon of water). check first to see if ur plants have a visible magnesium deficiency before adding epsom salt.
Magnesium is one of the macro nutrient, plants require magnesium throughout their life. Don't wait for it to show deficiency, add 1/4 tsp of epsom salt per 3 gal of water