If the original grammar is noticeably wrong and grates, I feel free to change it. After all, I'm a "folk" too.

But if the "correct" grammar sounds stilted or just doesn't work, then I'll sing the original.

For example, in Rod MacDonald's "A Sailor's Prayer" (here's an author-approved text), the chorus line

"I will not lie me down, this rain a-ragin' "

just doesn't work with "lay me down"; the original sounds good, wrong grammar and all, so I sing it that way.

But the last verse (in which "death waits just off the bow"), with awkward wording, seems to me such an artless mess, I sing it differently every time, and no one seems much bothered. (And if anyone did notice, I'd tell them what I did, of course.) Mostly, the verses are an excuse to sing the lovely chorus, in my opinion.