If she is old enough to hold it up in the air with both of her hands and control it, I see no problem...If this is propped up in her mouth where she cannot push it out, I do...Either way, I would watch her all the time...

my daughter is 7 months old, i never let her lay down with bottle. i always feed her head up. dont know how infants take it but at least i cant sip any drink while laying down. i mean i see it that way.

also i never let my baby feed in the car.

It should be okay as long as if you see her go to sleep to take the bottle out because if she already has teeth or will be getting them soon the formula will pool aroundher teeth when she sleeping and the sugar in the milk causes cavities, even with water. I give my 5 month old a bottle when shes laying down she seems fine with it, actually she likes it better that way she wont eat if i hold the bottle anymore.