The weather has radically changed in Western Pennsylvania during the last 48 hours.  The forecast is for a low temperature in the mid to high twenties till next Tuesday.  Many local nurserymen recommend burlap throws instead of plastic if you use an artificial barrier cold protection.  It is hard to predict what this cold snap will do to the rose bushes that have leafed out.  I doubt that it will cause any real effect unless the cold persists for several more days.  But if you removed all your winter protection and you want to brave the cold, replacing some of the mulch up around the plants would be a good idea.