Is it polite to ask about salary in a job interview? Why or why not?

It's not a question of politeness. It is most definitely something you should ask about. Salary is a component of the "package" (both the remuneration package, but also the "whole deal" - their culture, the learning opportunities, your satisfaction, the meaningfulness of the work, the hours, the products, the lot). There is a danger if you do not ask about it early that you are heading down a path that has a disappointing end. What if, after the second round interview, you think to ask about salary, and they nominate a number far below what you expect? Of course, you negotiate, but at the end of that, it's still lower. So low, you have to walk away - you have wasted your own time and emotional energy, and and the company's time. For this reason, we always list a salary on our career opportunities, and reconfirm the candidates expectations at each of the phone interview, first face-to-face, and second face-to-face. We try to create a culture of open-ness in the interviews, because sometimes, people do feel uncomfortable asking.

Though there is no   harm to http://blog.myjobhelper.com/2014/12/28/5-ways-effectively-negotiate-salary/in a job interview but I would personally   recommend not doing so. You can ask about the salary once you are in the   final phase of your job interview or if you are patient enough, wait till the   job offer is made.