I have a really hard time doing strict Pomodoro because I do find there are better and worse times to stop at for focus. I like this article and it's idea of being intentional about the stopping and starting point for the break. Often I take a break when I have the feeling I need to change my approach- stepping away for a bit and coming back a little fresher with some subconscious thinking having gone on. Forced interruptions are harmful, but intentional ones can be very productive.