I read somewhere that if the die is rotated, it can make the value of the coin go up. But when you turn the coin over, do you turn in side to side or up and down? x-axis or y-axis? Thanks.
Welcome to Cointalk! Side to side. You have it right if a heads up US coin, when turned, the reverse is upside down. (Unless a rotation error)
And significant (read that as maybe having extra value) die rotation starts at about 90 degrees. Extra value mostly depends upon the date. In general, 20th century coins with die rotation have a higher relative value than 19th century coins. That's because there was better quality control in the 20th century. [WARNING: As with all general statements, there are exceptions, sometimes many.]
