Great question and the easy answer is if it looks ugly, cut it back. This means fall-blooming perennials like dahlias, rudbeckias, and sedum Autumn Joy should be left alone to flower until the first hard frost. Then when they look black and dead cut them to the ground. Early blooming perennials like iris and daylilies that have finished flowering and are now turning yellow can be cut back by half to tidy them up. Anything with yellow leaves can be cut to the ground as the yellow color means the plant is finished making chlorophyll. You can wait until October and do the chop and drop letting the cut tops of perennials become a winter mulch if you don't mind a messy look or cut each plant down as it turns brown or yellow and compost the refuse if you want a neat and tidy garden. Either way, your perennials will thrive. The only plants not to cut back in the fall are tender shrubby perennials like hardy fuchsias and roses. This could stimulate growth and you want them to slip into dormancy. Keep growing, Marianne Binetti