Rhythm of speech gives your dialog flavor and your characters life and will protect you from the writing police who will quite justifiably have you arrested if you use dialect. People in Louisville speak in different rhythms than people in Brooklyn and people in California. They also say things differently. For example, my French husband has said to me: “Please to tell me what is in your head before you become the tornado.” There are no French words here, there is no dialect, no slang for that matter. But it is entirely French in the rhythm of speech, and in the actual words that he says.
So be aware that it is rhythm you aspire to, and that every book will have a rhythm and music all its own.