Q: Is it necessary to make the sign of the cross after receiving communion?
A: No. It’s very common, but it is not required. The Church asks conferences of bishops to determine a sign of respect BEFORE receiving communion, and in the United States our bishops have decided that this should be a bow of the head. Not a bow from the waist. Just a reverent head nod.
There is no recommendation in the missal to make any sign of devotion AFTER communion. Communion is the high point. No other sign can make a person more blessed.