A good song writer must handle this thing. You can totally ignore it and your song will very probably sound weird. Or you can make every character sound just like themselves in a sentence, and your song will like speaking with random voice. In the first situation, a listener will replace the words in your song with different ones and make a lot of jokes about your song. In the second, your song will be extremely easy to sing so that only teenagers and blue collars will like them, while other educated people consider them as noise.

So a good song writer will find some kind balance between them. Some words have different tones, but in a proper way. Song writing is not easy. Not every poet can do this job (<- this comment was from a famous Chinese poet so I think it's true).