I know residency interviews are wrapping up, so this posting may be too late for the current cycle. The only evidence-based guidance I could find comes from a brief 2005 report in Academic Medicine
. Unfortunately the sample size is small (n=27 gay medical students interviewed). Nonetheless, here is the salient finding in regards to your original post:
"During the residency selection process, 52% of medical students were unsure about, 33% planned to, and 15% did not plan to disclose their sexual orientation. Of those unsure, 37% were not certain if it was appropriate to do so, 33% would decide once they knew more about the program, and 22% once they met people at the residency program. Of those who planned to disclose, 26% wanted to determine if the program would accept their sexual orientation, 26% wanted to “find out the environment for people like me,” and 22% wanted to increase LGB visibility in the residency selection process. Of those medical students who did not plan to disclose, 60% were concerned they would not be accepted, 30% believed it was not relevant, and 10% were concerned that their medical school officials would be contacted."
Here is the reference: http://www.ncbi.nlm.nih.gov/pubmed/16043537
As you know, anti-discrimination laws protect you from having to answer certain types of questions. Ideally, the interviewer should know not to ask such questions. Here is a useful resource on how to navigate this often fuzzy area: http://www.uwec.edu/career/online_library/illegal_ques.htm
In regards to questions you should ask, this has been covered above and you must be a pro at asking questions by now given that you are far along the interview trail. Hope you match into your top choice in March.