Word. The 'T' is extremely "efficient" in getting from place to place, for sure. when compared to renting a car. You do not want to have a car in Boston - it's more of a liability/bitch/pain than anything else.
Also, staying 7-10 miles from downtown to save bucks as long as you're cool with the subway is worth it for sure. I am bringing my girlfriend to Boston in June, and it's her first time there/first time at Fenway....So I wanted to stay within the vicinity of the Tour-Trolley, because you pay like $30.00 for a ticket, and it has stops all over the main tour attractions of Boston all day. So it really works out nicely, you step out of your hotel, hop on a trolley, get off and visit Fenway Park or somewhere, get on a different trolley, go somewhere else, etc etc.
I was going to stay in Sommerville for a cheaper hotel-rate, but I didn't want to take the subway downtown, then catch the trolley for my stops, and then after the Night game(s) risk the T possibly being closed and not being able to catch a cab/not wanting to walk 8 miles with the lady.