I'm hiring a sax player to play on 3 of my tracks. I'm actually paying a studio to mix my album, and we decided to have someone play the synth-sax lines i had recorded. I'm not paying her at all, I'm paying the studio an hourly rate (for time, not creative input) and they are paying her.
For my expectations, i think signing off is overkill.
And if worst comes to worst (i become
popular or
respected), I do have the copyright and date-stamped recordings showing my playing exactly what she will be playing.