Hi unrivalled, sella..
Yes, you said it right! Rolling admissions mean first come first serve basis. Your application has to be
fully complete in order for the review panel to consider it for an interview. They will not even touch your application unless its fully complete!
Your complete application means official NBDE scores, TOEFL, ECE reports, notarized BDS degree, official transcripts, 3 recommendation letters have to reach office of admissions at BU (plus your online application).
So, a person with a 85-86 NBDE part 1 score ( coupled with other fair credentials) might get selected if he\she has applied to BU earlier in July-august. A person with a same or even better scores (88-89) might not get an interview call if he\she has applied late in the year (Dec-Jan).
Nutshell...need some good damn luck at that point

!
Hope this explains & Good luck ,
~ Pearl