There are different arrangements of the same song that use different chords. If you don’t know at sight what arrangement you are handing out to the band, then you could be in for a disappointment at rehearsal time. You should know based not only on the lyrics, but the chords as well.
VOCALISTS NEED TO KNOW THIS AS WELL. I can’t tell you how many times I’ve seen a vocalist attempt harmonize to a melody only to find that it clashes once they work with the band. Harmonized parts do NOT always strictly follow the melody. Sometimes they hold while the melody moves!