Google maps says Salem is 40min+ from Cambridge, and Nashua is 50min+ from Cambridge, and add another 10min to 20min from other parts of Boston.
Each minute of driving costs at least $0.67 (from IRS), excluding increased morbidity and mortality risks (injury from car collisions is the top health risk for most Americans).
So even using $0.50 per minute of driving, if you are only going to NH to evade sales tax, that is 80min*$0.50cents = $40. $40/0.0625 =$640.
So the first $640 of the purchase doesn't even save you any money (even more for most Bostonians further than Cambridge), and it costs you 1 to 2 hours of your life driving back and forth. If you value your leisure time at at least $100 per hour, then you're looking at spending at least $2,200 for the tax evasion to start paying off.
I'm just positing why Microcenter will not open a NH location anytime soon, because most of its customers (who are in Boston metro) won't find that it pays off to travel to NH.