Surely the context in which you are singing is everything? I've never sung professionally but have in various formal occasions such as choral evenings. In singarounds, I normally have a mental list of things that I may sing that are relevant to the time or date: ones about Napolean in mid-June for example. But I rarely stick to that because I prefer to react to what other people sing to make a kind of 'conversation' of the session. As a result, at sessions I sing from memory, even the ones I had as possibles. But when I sing a psalm solo in church, I always have the music. Ditto in choral work.

When I play music, I always have fixed pieces with the notes in front of me because I'm simply not confident (or competent) enough to do anything else.