Are we talking about things like troll subs with anime where somebody just plops down their own ideas instead of translating? Since I can't read Japanese I don't know how accurate translations are, but I figure that if someone goes through all the time and effort to typeset a bunch of bull then let 'em.
Some particularly amusing ero-doujin lines such as "Hideyoshi, your ass-pussy shudderin' down all on my cock feels the fuckin' cheese, yo!" almost certainly have taken liberties with the original dialogue, but for the most part I assume what I read is at least somewhat faithful to the author's intent.