Setting gain for vocals is important to a good sound, but also important for control of feedback.
Proper gain settings for vocals will aid in how well the vocals will "sit" in a live sound mix.
Gain settings should rarely be adjusted once they are set (as part of a sound check). However, small amounts of gain adjustment is acceptable to fine-tune a channel during a show.
-- DO NOT use the gain controls to mix your sound.
-- Gain controls should NOT be used as faders.