I don't want my computer to sleep when I'm downloading a large file. But I have kept my power options so that it sleeps automatically when not in use for 10 minutes. Some download managers (e.g. utorrent) do have an option that prevents the computer from going to sleep when downloading. Some don't (e.g. Flashget). Some downloads wont even resume after waking up from sleep. One way for the computer to sleep intelligently is Coffee.
Coffee is a freeware that prevents computer from going to standby when the network adapter is busy. The user can set the threshold above which computer won't sleep.