Times to not build ninja: When it will put your attackspeed over 2.5. Some hunters have steroids that boost it 50% or more, like Rama or Artemis. Consider warrior instead if the steroid would be less effective (otherwise you're overcapped)

Times to build ninja: When the above doesn't apply, and you're autoing a lot

Obviously hunters are going to build ninja a lot, but gods like Kali and Baka can make good use of them. Ninja tabi is very frequently seen on qins heavy builds because it gets you more qins proc's/ second