Under Robert's Rules of Order, the president is NOT required to abstain from votes on motions.  Unless an association's governing documents prohibit voting by the president, he/she may actively participate in board meetings and vote on all matters brought before the board.

PROCEDURE IN SMALL BOARDS. In a board meeting where there are not more than about a dozen members present, some of the formality that is necessary in a large assembly would hinder business. The rules governing such meetings are different from the rules that hold in other assemblies, in the the following respects: . . . The chairman [of the board] can speak in discussion . . . [and] can make motions and usually votes on all questions. (Robert's Rules, 10th ed., pp. 470-471.)

The exception to voting by the president is when there is a conflict of interest on issues affecting the president.