1. Yes.
2. It depends.
United has a policy not to check any baggage when there are at least 4 hours between the time you check your baggage and your flight:
http://www.united.com/CMS/en-US/travel/Pages/BaggageChecked.aspx (Under last section of "Preparing to check your baggage")
So when you check-in at Taipei, if the agent does not allow you to check your baggage all the way to Boston, your only options are either standby an earlier flight ($75 fee may apply) or your baggage stays with you when you visit San Francisco.
However, if the agent is able to check your baggage to Boston (not including the mandatory reclaim at San Francisco for customs), then you can drop your baggage at transfer. But keep in mind:
a. Your baggage may not be on your flight. United can, and will transport your baggage on next available flight, which your baggage will end up in Boston 1 day earlier (as you will arrive next day in Boston).
b. Your baggage has a higher risk to be stolen or damaged (the entire baggage or the content).
In this case, my suggestion is standby an earlier flight if you can spare time when you return.