I was wondering if one can regard a Sunni narration (such as one found in Sahih Bukhari/Muslim) as trustworthy in the light of the Ja'fari jurisprudence? I see Shi'i scholars use Sunni hadiths to make specific points.

I talked to a Shi'i brother regarding this, he said that it's best to disregard these whole collections as they might be biased. Thus even the most innocent looking narration about morality can be considered a fabrication.

Is this true?

Not necessarily.
If the narration is not against Qur'an and the proved sunnah, then it could be considered if the narrators are trusted.