A good HERS rater goes well beyond code compliance verification: they are an important part of the project team to aid in your success and the betterment of the building. Bring the HERS rater on early to make the most of their expertise.
The MA 2015 IECC is very similar to the current 2012 IECC. Blower door and duct leakage testing thresholds remain the same, prescriptive R-values are the same, continuous ventilation and high efficacy lighting are still required. If you have successfully built to the 2012 IECC, you are in good shape.
The new Stretch Code is a bigger jump that requires a robust HERS 55.
The ERI path is an alternative to the 'prescriptive' aspects of the code (see 402.1 table below) . This provides more flexibility for meeting code, not to mention Mass Save New Construction rebate opportunities (IMO, a no brainer for every new residential build).