If these are winter-tender roses that will be pruned severely in spring, that would get rid of the mildew on those plants. If they are hardy roses, they will carry mildew over the winter in latent growth buds on the canes to quickly infect new growth in spring, weather permitting. On hardy roses I would control the mildew now. A systemic fungicide would be most effective in preventing carryover, but various "organic" fixes will knock the visible mildew back and prevent the spread of fall mildew.