I usually keep mine one notch down from full sensitivity , the difference between that and full isn't very much but I feel the machine generally seems more stable that way. Sensitivity don't necessarily equal depth , especially when you are talking small adjustments.
In really trashy ground though , turning the sensitivity way down helps a LOT.... the signal from the coil is less affected by items around the outside edges of the coil giving you a cleaner signal on the good stuff. I will go all the way down to just one bar at times when the trash is thick. Going that low with the sensitivity will usually reduce depth significantly but in trash that thick you don't have much depth potential anyway unless you clean all the trash out first.