Hey Daniel, there's no rule, but probably most often you'll want the lower notes down first because that's the direction of a down strum.

HOWEVER, I'd recommend just getting them all planted at the same time. Caren, in her videos, recommends this little exercise -- forget the right hand for now, and just practice lifting all of your fingers off the A chord and placing them on the C chord at the same time. Just back and forth, over and over, no need to strum. A chord, C chord, A chord, C chord. Repeat endlessly until it becomes more automatic.

-Carl.