Pruning encourages new growth, especially side shoots. At this point in the season, if you prune, those little baby new tips won't have hardened off enough to survive frost. Now our average frost date tends to hover around October 20 or so, depending on who you ask. Will new shoots harden off in a month? Not really. If these tender darlings get zinged with frost, oftentimes they will turn black or brown. Now you don't want your yew bushes covered in brown bits just in time for holiday entertaining, do you? So back off.
See that flower bud on this viburnum? Put those pruners down!
Also, if you prune now, you could cut off the flowers of spring blooming shrubs especially lilac, viburnum, and forsythia. What's the point of lilacs with no flowers? I know that the landscaper crews are out and about trimming away, but trust me, sit on your hands. Have a cup of tea on the patio with the newspaper and enjoy the migrating birds.
This year, think about leaving up some of your perennials. 'Sacrilege!' you may say, but really, what are you going to look at through four to six months of winter? Great white sheets of snow? Yes, cut back the perennials if they have had fungal problems. This year, peonies have taken a beating from botrytis, so it's best to cut them back and destroy the foliage. If your foliage is clean, though, leave it up. Not only are seedheads attractive in our frosty winter, but they feed the birds. We have left even basil to seed and the juncos thank us for it. There's a whole patch of hosta that has reseeded thanks to the chickadees. Also, dried foliage will help insulate the crown of the plant during our famous frost heaving. In spring, I tend to clip remaining material and let it compost where it sits. I also like to see where plants will reseed such as columbine, hollyhock, and campion.
If you have to do something in the garden - weed. Pull those bad boys before they go to seed and as they are sending nutrients to their roots. A thorough fall weeding can save you a lot of time in the spring.