Water them! However you've been watering them during the hot summer, it isn't officially fall until the Autumnal Equinox, September 21st, give or take a day.
The "trimming" rule is to prevent roses from putting out young, tender growth just in time to have it killed by early frosts. When is the first hard frost expected in your area? Work with that date.
Are you talking about pruning the roses, cutting back live canes? That may be work for the spring in your area; check with a local rose grower. Or are you thinking about dead-heading, cutting out dead twigs and branches? That can be done any time.
Sunset Zone 17, USDA Zone 9