My own practice is to quote a quote verbatim, punctuation and all. If the punctuation in the original is so bad as to introduce genuine confusion as to what it means (as opposed to a use of punctuation that merely isn’t my, or my “house’s”, preferred style), I would either insert a bracketed comment (possibly even a bracketed punctuation mark) or break the quote up into two or more pieces to avoid the punctuation-related awkwardness. I try to avoid [sic] if I can, since it can come across as snarky. But if I need to give a lengthy block quote for one reason or another, and there is a glaring grammatical error in the original, I would use [sic].
Example: hypothetical original (verbatim) quote of an e-mail from “Bob”: “My favorite books is the Bible, the Constitutions and anything by that King guy.” (Assume that Bob was referring to the US Constitution, not to the constitutions of various countries.)
Any of the following would be OK (some more so than others):
Bob reported that his favorite books are “the Bible, the Constituion and anything by that King guy.” (Note the non-correction of the non-use of an Oxford comma, even though I strongly prefer them. If it wasn’t clear in context that the US Constitution was meant, I would probably add a bracketed comment for that, too.)
Bob noted that his favorite books include the Bible, the United States Constitution, and “anything by that King guy.” (Preferred method if I am trying to capture some of the flavor of Bob’s speech, but do not want to ridicule his grammatical errors. Here, it is perfectly OK to use an Oxford comma since i have not put that part in quotes.)
“My favorite books [are] the Bible, the Constitution and anything by that King guy,” Bob noted. (Ok, but the brackets seem to call unnecessary attention to the corrections, and, in turn, to the implied errors.)
Bob wrote, “My favorite books is [sic] the Bible, the Constitutions [sic] and anything by that King guy.” (Most snarky, and least favored, approach, unless there is some legitimate, non-juvenile, reason to call attention to his grammatical errors. Then again, if it’s clear from the context that I am using the quote for the express purpose of giving an example of his use of non-standard grammar, the [sic]s would be unnecessary, I think.)
And of course, I could just avoid quoting all together:
Bob’s favorite books are the Bible, Constitution, and the works of Steven King.
Although I don’t think I could resist quoting the “anything by that King guy” part, since in injects a bit of life into an otherwise sterile and banal statement. Also “works of Steven King” may imply that Bob has a greater degree of familiarity with King’s body of work than I suspect is the actual case.
Apparently, though, there are some newspaper articles in which quotes were “corrected” to make them match the applicable house style. I don’t think that’s appropriate. It’s not clear to me, though, if these were products of the newspapers’s official policies regarding such things, or the results of overly aggressive editors (whose work went un-re-corrected), or just unconscious “correcting” that occurred because the house rules had become deeply ingrained.