As a Leafs fan personally I don't believe the Bruins 'tanked' any games and think it's a fairly ridiculous narrative. Sure, the last couple of games weren't particularly meaningful so I could see the Bruins playing at a lower gear to prevent injuries etc, but that's no different than any other top team that pretty much had their playoff berths locked up long ago. Also given that the Bruins played the lowest playoff seed last year and blew a 3-1 lead, I can't imagine they would underestimate any opponent and actively try to throw games to ensure a 'weaker' opponent. This is hockey, anyone can lose.
Also, if the Bruins were really trying to tank, making light work of a desperate Pittsburgh team trying to make the playoffs while dropping a close game against another desperate team on the verge of locking up their playoff spot in Washington and then losing to the Sens who've had nothing to play for at all for months in the final game of the season is a very roundabout way of doing things. Pretty sure the Bruins also dropped 20+ shots on the Sens in the third period as they tried to mount a comeback attempt.