Textual analysis: The "nevertheless" in verse 6 is mildly confusing in the context. That transitional word would usually indicate some type of contradiction between the text which follows and that which preceded. In this case, that is not obviously so. Verse 5 says "1 Ne. 19:5 And an account of my making these plates shall be given hereafter; and then, behold, I proceed according to that which I have spoken; and this I do that the more sacred things may be kept for the knowledge of my people." Thematically, the keeping of the sacred things which is the ending theme of verse 5 has no contrast with verse six's "I don not write anything upon plates save it be that I think it be sacred."
If there is a contrast for the "nevertheless," it must be to a previous clause in verse 5, most likely the "I proceed according to that which I have spoken." Nephi's narrative to this point in the text has largely focused on himself and his own actions, with the historical information about his family interwoven in the procession of Nephi's spiritual development. It is possible that Nephi's humility dictated that he qualify his apparent self-indulgence (in relating his own story) by noting that the focus is on spiritual things, and not upon his own greatness. The very next sentence appears to support this idea as Nephi then declares his essential humanity and fallibility. While Nephi wants to make it clear that this is a sacred narrative, he also wants to make it clear that it is a narrative written by a man, subject to human error.
Scriptural analysis: Nephi declares that he might err in writing this history. It is interesting that he would so indicate, and that he would place the blame for any errors on his fallibility. What errors might be in the text? For one thing, a lack of completeness. Nephi's concern over spiritual matters clearly (and explicitly) overrode his more historical concerns. Thus we can expect that the historical record we have received through Nephi might not be complete, as he had no real intention of creating a complete record.
What else might be in error? Because of Nephi's powerful spiritual experiences, it is unlikely that any of the information Nephi attempts to communicate might be in error. It is possible, however, that some of the words he use might be less than idea (particularly for an English audience who receive the text in our language and not in the language Nephi used on the plates). The "nevertheless" in verse 6 is perhaps an example of a construction that is more difficult understand than perhaps it could have been.
None of this should or can detract for the power of the spiritual message. Nephi's declaration of his humanity and fallibility should remind all of us that even the prophets begin just as we are, and that should give us some confidence as we progress toward more knowledge.