For me, I found I learn and retain lyrics much better if I never use a lyric sheet in the learning process. If I start playing/singing the song using a lyric sheet, it takes longer to really know the words and not need it. (Haven’t used one in 10+ years, and when I did, it was only during practices, never live.) I learn lyrics by listening and singing with the tracks while driving. Currently have about 70+ songs at the ready.