If you can manage it, I highly suggest commuting to TF Green first and taking the T from there if you're close enough. I commuted from TF to Back Bay constantly at my old job and it honestly wasn't bad at all. Especially if you can entertain yourself like I managed to with audiobooks or my kindle. It's a guaranteed seat from there in the AM, too. Took a 7:30(?) train which got me in by 8:45 iirc.

The majority of other folks in my office were beheld to commuter rail schedules as well, you should be able to safely assume your office will be too if it's in/near Back Bay. At my office it was pretty much assumed across the board that everyone was going to arrive at some point between 8:30 and 9:00 and I was never questioned about it.

Back Bay outbound during rush hour though is as much of a madhouse as the rest of the inner-city stations, so just be prepared to have to fight your way through to the doors as soon as that train stops at the station if you want a seat. If you can't manage to find a seat, you're standing in the isle for at least the next 30-45 minutes until you reach Mansfield which always was a bummer.