Thanks for the input.

DD1 will begrudgingly fall asleep in the car without one, so I guess that's a start.
She's always slept extremely well, so I hate the idea of possibly messing with it! On the other hand, she was extremely attached to her sleeping bag and I thought we'd never get her out of that, but when she moved into a big girl bed with Dora everything, she was happy to go without it....