I have to say that grammar is for English classes, not songs. The thing people lose sight of is that grammar is, in fact, an artificial construct which was created after the fact. When the English language evolved, nobody was creating rules to be followed. Rather, what has happened is that much later, as in literally hundreds of years later, a small group of educated people decided to codify the language. Understand that I have no complaint with this, I have spent a lot of time reading very old documents which where pre-codification and it is very nice to have some rules that everyone follows. Still, these rules have far more to do with the printed language, not the spoken language. That is why we have dialects and regional pronunciations and colloquial expressions.

The point here is, songs are a part of the oral traditions much more than the written forms. Even today when they are written down and recorded on CDs and such, the main character of a song is that of oral tradition. If we try to limit this to a set of rules created in the sterility of the university chambers then we take away the most important values of the song. Personally, I sing songs with not only the grammar that it was written with, but I even sing them with the accents and vocal character of the artists who I associate with the song, who may or may not be the person who wrote it. By vocal character I don't mean I try to imitate the artist but rather my voice tends to take on those characteristics that identify the region the singer comes from, and even the social and economic class. To me, all of this is a part of the song as much as the music or words.