The 'witch' and I pretty much always used songbooks if we had an actual audience - oddly enough, though I'm the one with the chronic memory problems, I was also reliably the one who knew the words! We mostly used the books to a) remind us what songs we knew, and b) sort out what order verses came in & - as others in the thread - make sure we were both on the same version...

But on the other hand, we could (and did) do most of our regular repertoire walking down the street with no aids whatsoever, just with the occasional wrong-verse stumble or 'la la la something' line substituted...