I fertilize some of my perennials in late summer. My thinking is that we've got about 2 more months of warm weather, which gives most new growth time to harden.
With roses, I continue fertilizing until the end of September.
We get our prettiest roses in the fall and I want them to be as big and beautiful as possible. I know that most gardening books advise not fertilizing this late in the year, but in our climate and with our sandy soil, roses seem to like the extra fertilizer.