Most dental students apply for advanced education programs during the summer and fall of D4 - not after D2 as charlestweed's post might lead you to believe. Some schools have their students take the NBDE Part 1 during D3.
Of course, you could always apply at any time after graduation. Many students work for a few years or complete a GPR/AEGD residency before applying to a certificate specialty program. But no program would offer acceptances before your fourth, or final year.
Although, there is certainly importance placed on GPA (where applicable), Class Rank (where applicable), and Board Scores - the process is dynamic and quite complex, much like dental school admissions. The numbers aren't everything. Not every dental school features the same curriculum and regulations. And there are always students that get into the highly touted, competitive programs (Oral Surgery, Orthodontics, etc.) without 90+ Part 1 Scores and without being in the top 10% of his/her class. With the NBDE shifting to a P/F system, the Pass/Match process will probably become even more holistic.
It's never taboo to talk about specializing. Many people become initially interested in dentistry due to one of the many specialty tracts. Experiencing general dentistry is not a prerequisite to establishing interest beyond that of a GP. It would be a tremendous generalization to assume that most people, prior to dental school, only have applicable experience through shadowing or reading information online. The legacy dental student is still quite prevalent in many classes - and those that are son/daughter of an Orthodontist, for example, will come in with substantial knowledge and may very well follow through on an early commitment. That being said, in general, you should always keep your options open and not prematurely bias yourself towards any one thing exclusively, especially as a pre-dental student.
Also, regardless of whether you think it's "okay" or not, there is nothing wrong with an individual pursuing a specialty tract due to financial considerations or public perception. Everyone has different reasons for following a career path.