Depends for what your clients pay you for. My clients pay me for my time, not the code. I have used code written in my own time for client projects and I have u

Depends for what your clients pay you for. My clients pay me for my time, not the code. I have used code written in my own time for client projects and I have used code written for a client for another client's project. I also do some custom work for customers of our saas and I always tell them that the code I write for them is non-exclusive which means I get to re-use the code and resell it. I estimate nearly 80% of the app has been created that way including almost all the add-ons.