The theory is, (I think) that if you cut the foliage now, you remove any tatty bits and can see any problems before they get worse, such as fungal diseases. Then the plant has a chance to produce a nice batch of fresh leaves, which should last through the winter, and need tidying up again late winter. But if you have just a few plants, and some are small, maybe use your discretion and do some snipping, but they may not need the shears just yet! If you use shears, best to be careful not to cut too low either, as you may damage the crowns. I tend to shear the older plants, and snip at the younger ones.
