Although many Open Source projects don’t require you publish the modifications of them, I still propose you should make the changes open. The reasons are as following:
(1) For other people: the out-of-box Open Source projects save your time and money, and you get benefit from them. You should not only take, but also need to give! Only if everyone shares his code, document, or whatever related, the Open Source projects can grow up healthy.
(2) For yourself: You release your code can actually make you “richer”. With more and more people use your code, more and more bugs will be found, and associated patches would also be submitted simultaneously. In fact, you can leverage the whole community to help test your code and it saves your bucks which could be used to employ QA/testing engineers. The other invisible gain is the contributions can boost both you and your employer’s reputation, and it will definitely be a big fortune in your future life!
Based on the above points, don’t be hesitate to share your changes. You just need to know give more, gain more!