I would shadow
all the specialties for a few reasons but for one in specific. You won't know who will be interviewing you at a school you have been invited to interview at and what speciality he/she practices.
At a lot schools you will get one faculty and one student or at least one faculty interview. At one interview I had a faculty who was a Periodontist. This was the only speciality that I had not shadowed

. So right from the get go he had a preconceived notion that I didn't have interest in his profession. It made for an interesting interview explaining why I hadn't shadowed one.
However, the interview went great, but I have a feeling it would have gone much better if I had shadowed his specific speciality. Just learn from my mistake I didn't think would be an issue and shadow every one.
-General
-Oral Surgery
-Ortho
-Endo
-Perio
-Pediatric
-Pros (not as common, but you never know)