Dividing them would probably be beneficial ultimately, but if they are still blooming reliably, not absolutely necessary. You can shear back the foliage to the ground in November/December, and they will push lots of new foliage and look fresh and full again by spring. This presumes you will have enough fall winter rains to encourage fresh new growth. If you have them on irrigation and rains don't cooperate, give them enough to encourage new growth. Adding compost as mulch around them will also help.

THANKS Bahia for your advice to do the shearing and watering later; always happy to have a reason to procrastinate on big jobs when there is so much fun to be had in the autumn months.
Of course in December I know I will wish it was already done. :) Min