Don says: "... sometimes my mistakes DO become part of the arrangement!"

This is true. And therein can lie the problem. While some mistakes have wonderfully serendipitous results, I think more often than not the improvised lyric is inferior to the original -- at least if it was a superb lyric to start with.

As for whether one would/should drive a car with a book or printed sheet in front of him/her, hey, lots of drivers have GPS mounted on their dashboards for them to look at while driving -- along with the rest of the instrument panel. ; )