Jim switched to SS because a lot of people were complaining about the brass. For what reason, I have no idea.
No, it's not a matter of lead in the brass, it's because the two base metals that compose brass, copper and zinc, can output potentially dangerous byproducts when exposed to too much heat, and those chemicals can also leech into juice if the liqiuid is in direct contact with the brass for extended amounts of time. Considering how little brass is actually at play here, the fact that the + post is exposed to so little heat (unless you have a short or your wick is touching your post, neither of which should ever happen, brass or no brass), and how most of us were inhaling 20+ cigarettes laden with 400 different toxins and carcinogens a day, I don't think the brass nuts are anything to be worried about.