As to trim, WW2 fleet boat doctrine says this:
"Submerged operations are usually carried on with the submarine in the state of neutral buoyancy."
http://www.maritime.org/fleetsub/chap18.htm#18B
Read the above link, but several sentences down:
"The state of
exact neutral buoyancy is probably never attained, but the approximation is near enough to allow depth control to be exercised easily by the diving officer. In all normal submerged operations, the submarine is underway at relatively slow speeds. This horizontal motion through the water enables the surfaces of the diving planes to correct the effect of any slight positive or negative buoyancy and also to increase or decrease the submerged depth at the order from the conning officer."
Now, at least in my cold war days, a slight negative buoyancy was preferred I suppose, because at periscope depth, you'd rather sink vs broach and flap your fairwater planes at the Soviets - in the event of a loss of trim control.