8-10 ranks deep for me. Very effective against cavalry but of course the unit has to be standing still.

Also I increased the default depth for the A.I. with 6-8 ranks for most infantry and the A.I. comes out with increased morale and units that stay in the fray longer.

So depth definately adds morale. I don't reccomend less than 5 ranks deep for most infantry units as it can lead into the unit getting broken into 2.