Sounds like you are raring to get out there in your garden - I hope your back feels much better now. The best time to divide most perennials and transplant them is in the fall, but you can wait until early spring to transplant your garden phlox (Phlox paniculata). You should dig and divide them every two to three years to keep them vigorous.
As to those perennials - Cut away. You can cut back most perennials to within a few inches of the ground. And it is not strictly necessary to do a big fall cleanup of perennials, unless you don't like the way certain ones look in winter. You can always go through and clean up in cycles, too, as plants become unsightly, starting in late summer.
--Stephanie Hainsfurther, Albuquerque, New Mexico (a former New Englander)