Unless you are memory trained, then with age memory fades. Writing the songs down in a family book, for that reason is an accepted Tradition.
My wife is a Romany Gypsy, she writes her songs down in her very best handwriting in her songbook, and then sings with her eyes closed! (Figure that one out!)
That said the Gypsy Folk mostly could not read and write. If you look at their songs they sometimes tend to be confused.
The great Traditional singers Henry Burstow sent a list of the titles of his songs to Lucy Broadwood, and also sent some of the words as well. It was Bert Lloyd who pointed out that Folk songs tend to survive in better form with the literate.
So reading the words if necessary is not such a crime, however, not practicing or working upon your singing style is a transgression in my book. So the question that should be asked is why are you singing in the first place? The answer should be, to make my audience love this song as much as I do. So if you're heart is in the right place who cares if you need a memory aid? I don't yet. However the time may come.....