The rubrics specified in the General Instruction of the Roman Missal specifically instruct the communicant to bow his head before the sacrament as a gesture of reverence, so it is required for those physically able to comply.
When receiving Holy Communion, the communicant bows his head before the sacrament as a gesture of reverence and receives the body of the Lord from the minister. The consecrated host may be received either on the tongue or in the hand, at the discretion of each communicant. When Holy Communion is received under both kinds, the sign of reverence is also made before receiving the precious blood. (GIRM 160)