Here is another fascinating study by the secular humanist group CFI on Secular Identity among College Students. The entire article is worth reading, but here are a couple of paragraphs on self-identifying as atheist:
In order to ascertain their worldview, we also asked students to choose whether they would describe themselves as Religious, Spiritual, or Secular. The Secular were a heartening 28 percent of the total, only slightly less than Religious (32 percent) and Spiritual (32 percent). Nevertheless, there were more Nones (33 percent) than Seculars (28 percent) among these students. The Nones split 70 percent Secular to 30 percent Spiritual. This meant that 70 percent of the Secular worldview group was composed of Nones and 32 percent of the Spirituals were Nones. Why this discrepancy? It appears to reflect the plurality of females among the respondents—women who self-describe as Nones tend to avoid the Secular label and prefer to identify as Spiritual.
Identification patterns are changing, and young males seem to be much more willing than older generations to adopt the atheist or agnostic label. As a result, around 28 percent of those in the Secular worldview group self-identified as atheists and agnostics. This might be seen as progress, but figure 1 shows that when asked a theological question about the divine, just 77 percent of the Secular group provided atheistic or agnostic responses. Again we find a discrepancy, this time over the atheist self-designation: What does it mean when 42 percent of Seculars provide an atheistic response to a God question but only 12 percent self-identify as atheists on a religion question?
I think you are right about exercising discretion until you know someone. In our society, some very religious people will “shun” an atheist, and some will feel compelled to “witness” to them, which easily devolves from friendly debate into bitter argument. In most contexts, better to keep your opinions to yourself until you know how a person might react. It’s what my atheist friends do, and probably what I would do in their shoes. This also figures into the above stat on young males. They’re not tired of confrontation yet. Haha