In short, yes, it’s okay to stand to sing during an LDS sacrament meeting service. I’m never sure where these weird rumors get started — I blame the Utahns.
Congregational Singing
Most Church meetings are enhanced by the singing of hymns. Music provides a primary means by which members participate in Church worship services. Congregational singing has a unique and often underused power for unifying members as they worship together.
As appropriate, a priesthood leader may ask a congregation to stand for an intermediate hymn or a national anthem (see “Hymns for Congregations,”Hymns, 380–81). (Church Handbook of Instructions, Book 2, Section 14.4.3)
Read the Church Handbook of Instructions yourself here: https://www.lds.org/handbook/handbook-2-administering-the-church/music?lang=eng (Once you get to LDS.org, you’ll have to navigate to Section 14.4.3 using the menu on the left side of the page because the hyperlinks don’t work.)
