Please see comments below. This approach is kept for historical reasons, but is not necessarily recommendable.
Kind of new on hermeneutics.SE, but I have had the power for a bit on mech.SE and obviously have the power on Internet of Things (a site I moderate.)
From my perspective, protecting is best done as little as possible. In other words, if it's a question that attracts new users - great! Get them on board. In the beta period, we need more users who will stick around, post, vote, ask, etc.
Personally, I will definitely not protect a question before it has one deleted answer, and usually not before it has two. Bear in mind that protecting is essentially restricting access.
It happens from time to time that I visit a site where I do not have the rep to answer protected questions and see a protected question that I feel I could really add something to. Unless I see that I really, really have something important to add, it's rare that I'll bother sticking around to get the extra reputation required. Rather, I just move on and forget all about it.
On the other hand, if a question is generating a large quantity (I tend to go with 25-40% of answers) of NAA / VLQ flags, protecting just makes moderation easier and keeps the question cleaner. In that case, I personally say, "Protect!"
N.B.: This is just my personal bent; feel free to disagree.