I'm trying to read through the collection of essays called Revisiting the Corruption of the New Testament in preparation for a talk I'm giving in a week. The second chapter caricaturizes Bart Ehrman's text critical rule as "The least orthodox reading is to be preferred." The claim, which seems generally valid, is that Ehrman functions, for all intents and purposes, with the sense that if when you are faced with variation among manuscripts where one text has wording that is "orthodox" and another has a wording that is problematic for faith, then the problematic one is more likely to be original.
I want to start off with my conclusions. First, Ehrman does seem to function in this way. It is his consistent practice. Second, there is actually a kind of common sense to his practice, although ironically he probably is wrong on some of his conclusions on individual texts. Thirdly, it is completely irrelevant to Christian faith. This is an issue of no importance whatsoever to Christian faith (which is why, sadly but understandably, our streamlined 75 hour MDIV at Wesley spends all of an afternoon on the question of the text of the whole Bible, the cost of majoring on the major).
The real rule, set forth by J. A. Bengel (1687-1752), is that the more difficult reading is to be preferred. I'll put it a different way. A good decision on how an original text read has a good hypothesis on how the other variations in the manuscripts arose. The idea that later scribes intentionally made manuscripts less orthodox makes little sense, while Ehrman's hypothesis--that scribes intentionally or inadvertantly made the text more orthodox--is a more likely explanation in theory.
A less orthodox reading might be a "more difficult reading" and therefore a more likely original reading. But interestingly, in my opinion, it doesn't turn out to be the case with most of Ehrman's key examples. Orthodoxy doesn't seem to have been a primary factor in the copying practices of scribes (harmonization of the details, interestingly, was much more significant).
We need to get concrete probably for this discussion really to make sense. Did John 1:18 originally say, "No one has ever seen God, but the only begotten, [who is himself] God... has made him known." Some manuscripts read here "the only begotten, a Son... has made him known." Ehrman believes that a copier of John intentionally changed "Son" to "God" to make the text fit better with orthodox theology.
Is this possible? Sure? Does it matter? Not in the slightest as far as Christian theology is concerned. John 1:1 already identifies Jesus as God and even then, we have to interpret what that means. If single verses cause you to break out into faith sweats, your faith is out of focus. This goes to a contention I've made elsewhere that Ehrman's childhood fundamentalism is still setting the agenda for him as a scholar today. Me? I'm good with either reading.
To put on the scholar's hat, though, I think Ehrman is probably wrong on this one. I consider "the only begotten, God" to be the more difficult reading. For one thing, "only begotten, the Son" is very Johannine. It's the kind of reading we can see a copier accidentally putting down because they weren't paying close enough attention. There's no theological issue with "only begotten Son." It fits fine with Christian theology. A scribe wouldn't have found it theologically problematic in the first place.
The "external evidence," the lay of manuscripts, soundly favors "only begotten, God." The second century papyrus p75 has it, as do Vaticanus and Sinaiticus from the early 300s. In other words, the "oldest and most reliable" manuscripts in this case support "only begotten, God."
Did scribes ever make texts more orthodox? I'm sure they did. It's possible, for example, that Mark 1:41 originally read that Jesus was angry rather than compassionate. The chapter has a nice footnote list of possible examples (88, n.129).
Does it matter for theology? Not in the slightest. And it's not as if there isn't enough evidence to sort most of it out, even if one does have some neurotic fixation that compels you to know for certain what the precise wording likely was. But this can approach bibliolatry at some point for a fundamentalist--or neurosis if you are an ex-fundamentalist doing his own version of scholarly therapy.