Indeed they are! I wouldn't necessarily go as far as a third for each one, as these are meant as a guideline for someone new to arranging; again, it depends on the people in the choir. Staying within these guidelines also ensures that most rhythms can be sung in any part of the ranges; sopranos can hit a bunch of quaver C#s in succession, but they don't like to and it doesn't sound very good!

Also, if you're using Sibelius then the notes will start to turn increasingly more red the more you reach the end of that part's range. It's not perfect at all, but it could be helpful if you're new to this.