Roses like to be feed continually. If the customers wants non stop blooms then they need to look into light foliar feeding them. If they do not want to make it too complicated on themselves the Bayer all in one plant food/fert/pesticide/disease products at home depot/walmart/lowes do wonders.
We actually have companies down here that specialize in rose care....nothing else. If they want real showy roses, just like anything in life that equals higher inputs. Properly selectively pruning will also help promote new growth and better health.
IMHO even the "Carefree" roses are not so. It has been my experience that feeding roses with a product called Eagle 6 is one of the best products that I use to promote very healthy happy roses . Here is a link in case you are interested. http://plantfoodsystems.com/wp-conte...e-6-8.5x11.pdf