what's great about iOS, is that you won't notice a performance slow down even if you only have 50MB of available space. it doesn't act like a conventional operating system, especially not like Windows where when you get really low in storage space so many crazy things start happening error wise.
i once got my iPad down to 24MB remaining. no performance slow down whatsoever. however, as someone else mentioned, i couldn't update an app. no biggie - i deleted something and made room for it. you can do this hundreds of times and not see a reduction in performance at all.
i truly love iOS with a passion (coming from a former Blackberry owner)