oldtangoguy, you posed a good question. The lyrics (if you know them) can inform your dancing interpretation of the song to some degree, or help you understand the music better. I offer my students musical studies with lyrics to help them understand the mood of the piece.
BUT... 1. Unless you understand Spanish, you can't possibly be influenced by the lyrics. 2. You need to know ALL the verses ahead of time to get the poetry. 3. As you pointed out, some happy sounding songs have sad lyrics. (although this opposition is sometimes counterpoint).
I pose that it is best to dance the energy of the music and the tone of the singers voice as you hear it. This approach allows better improvisation, responds to the changing emotions, and is easier for the average dancer.
PS, one song that I like with changing moods is Esquinas Porteñas. The stanzas instrumentally reflect the happiness that once was, contrasted by the sadness in the refrain. The contrasting lyrics are not in exact time synchronization with the music´s contrast pattern, but poetically matches.