In Minnesota, there really is no law that says you can't. According to the Digital Media Law Project, there is no statute that specifically address recording inside a polling place. I did a quick look as well and the law says basically to vote and get out. The Office of the Minnesota Secretary of State strongly discourages voters from using cameras or video recorders in the polling place. The Secretary of State's guide for election officials also notes that party-appointed challengers should not be allowed to "take pictures within the polling place." (See p. 16.) http://www.dmlp.org/state-law-documenting-vote-2012#Minnesota
However, if you are in Florida, Georgia, Kentucky, Nevada, North Carolina, Texas, West Virginia or any other state that has a statute or written “Official Statement” that specifically prohibits recording in a polling place then you could be in some hot water for photographing your ballot.
Some of the concern is that candidates could offer money for votes. Photographing the way you voted is proof that one votes a certain way and then money can be paid for the vote. Other concern is privacy and some is just government regulation.
Should you do an election selfie? An elfie perhaps? Yes, with your little red sticker.