They live all over the place...Quincy, South Boston, Dorchester, Harbor Point, Malden, Kenmore Square, Malden, Copley, South End, Symphony, Mass Ave, Chinatown, etc. I think the school will hand out a list of those looking for roommates.
My first 2 years I lived in the symphony area. There are 3 major bus routes nearby - the 1, CT1, and BU Shuttle. The best restaurants/shopping nearby. The nicest and safest area in my opinion to live for BU Dental/Medical students.
The last 2 years of dental school I lived across the street from the medical campus in James Court. Same price, but brand new. However, I never liked hearing all the gunshots multiple times a week, and I really hated all the pulled fire alarms in the middle of the night. At one time our building had 6 alarms in 1 month, and it can take over 2 hours for the fire department to turn off the alarm.