In which case you make spheres, not cubes, as spheres have the lowest ratio of surface area to internal volume, thus reducing the ratio of hull armor to total ship mass.
In addition, the entire external hull benefits from the "arch strength" principle, making a hull with exceptional strength for the mass.
Depends on whether I'm flying a transport or a warship, I think. Also depends on the battle tactics planned. If I'm expecting to trade fire at long range using beam weapons, a torpedo shape with an up-armored nose and stern might be better - presents less of a target. Wedge works too - you're hitting the armored surface at a sharp angle, therefore having to plow through more armor to get at my soft innards. I see sphere as a "bar-fight" kind of shape: don't know which direction fire is gonna come from, so protect from all directions and maximize your ability to pivot and change direction quickly. Maybe better if you're expecting to receive a lot of missile fire.
Getting back to the butted spheres thing, I can think of only a couple reasons one would want a multisphere ship like that:
1. Aesthetics: Look cool, attract customers, make money.
2. Need: you want to be able to easily isolate one sphere from the others - as in a prison transport, bioresearch vessel, a ship with the kind of drives that might need to be jettisoned in a hurry, that kind of thing.
I recall once seeing an interstellar "slowboat" design, sub-C, big spherical fuel tank up front with the rest of the ship strung out on a boom behind that, smaller living section about midway down the boom, drive section farther back, the idea being that the big sphere acted as a shield at near-c speeds with the other sections basically in its shadow. I don't think the artist thought it through - when the ship turned about to decelerate, the sphere would then be trailing instead of leading. However, two spheres with the living section between and the drives aft of the second sphere would do that job okay.
