I graduated college with a 3.591 GPA...would it be terrible if I rounded up and put a 3.6 on my resume when applying for jobs?
Yes, it would be wrong, unprofessional and unethical. You would be misrepresenting yourself.
It is mathematically valid to round it to 3.6. And in this case, it would be more transparent to do so. That is because three places of decimal do not convey as much information to a reader as one in a social context; a mathematical context is different.
If you know the rules of rounding, you wouldn’t be asking that question.