Regardless if its a free or premium theme, testing should be a major part of your theme development. Through this, you can detect and deter bugs, and helps you prevent other mistakes. This article will give you the steps to go through the process of testing WordPress themes.
Although testing is just simply running a theme and looking for bugs or errors, it simply won’t cut it. You have to know what steps to take, separate each steps into distinct sections; so testing a theme is now more accurate and thorough.
Read the rest of the post here: http://wp.smashingmagazine.com/2013/05/29/tips-tricks-testing-wordpress-themes/