Tangential fact - while, as you described, most popular songs in Cantonese have tones matching the melody (usually the melody is written first, then lyrics are filled in), many songs from Christian churches don't follow this practice. (I don't know why, maybe a lack of lyricists for translation from English/Latin to Cantonese during earlier years?)

So, in Hong Kong, when somebody writes a song/lyric that doesn't quite have matching tones, we ask "which church are you from?" to make fun of it.