I find it impossible to sing a song with feeling unless I know the song inside out.

Having heard singers "reading" words of a song for what appears the very first time I find it difficult to put the effort in to listen!!

Having said that I have the first word of each verse of some long songs on a crib sheet as an aide although I often don't look at it. I only do this with songs where I feel it would be tragic to forget the order of the verses.

For me the better a singer "knows" the song the better it will sound; with or without the words.