I’ve lived here on and off between 2012 to present day, and Manchester really isn’t that bad. I’d say the rougher areas are mostly residential with convenience stores, laundromats, etc. you likely won’t be patronizing anyway. Basically, between 10pm - 6am, I’d just avoid walking alone in the area of pine street to maple street, between maybe orange street and somerville street (pine through maple streets run north to south, parallel to elm street, and orange through somerville streets run east to west, perpendicular to elm street), which is easy enough to do.

Enjoy your new job!