From: The House of the Mouse
That's why the dawn assault was favored- the 'ideal' is to form up, and close with the enemy while its dark, so the artillery can't get you, and have dawn break right when your units come face-to-face with the defenders when they are too close for the defending artillery. Also using your artillery on the defenders to suppress them will help some.
Yeah, I noticed a problem with setting Move (with attacks) orders is they can be a little disjointed. The troops get there, start being shot at and continue being shot at whilst the commander has a think, then replans, then reorganises for an assault, and so on. Add in the possibility of the time and life cost of a company trying a failed assault first. At any rate, it wastes vital time and the lives of your men. If you know an enemy is there, issue Assault orders.
My main problem with issuing Assault orders is that the AI absolutely loves dropping artillery fire on assaulting units. I've lost count of the number of battalion-strength attacks I've had single-handedly crushed by shellfire routing the line companies one after the other. The AI has more trouble stopping a whole 3-battalion regiment assault with arty, but it can seriously damage plenty of it.