Is a switch hitter required to declare a side before batting?

Not in MLB. A batter may bat from whatever side he wants to on whatever pitch he wants to. The only restriction is that the side the batter is on when the pitcher steps on the pitching rubber is the side the batter must bat from for that pitch. A switch hitter could alternate sides for each pitch if he so desired. Rules concerning the batter can be found in Section 6 of the Official MLB Rules.