Space is very big, but by the same token it is also mostly worthless. Controlling a sector of space does you no real good, nor your enemy any harm. It's much like the sea in that regard: in and of itself it is valueless. What it gives you access to, however, may be very important.
However, there is a corollary: FS ships are not required to "cruise" through space to reach a destination. They can use intrasystem jumps to reach other places instantly, and without the enemy being able to stop the transit by any means we know of. This means that space in its entirety is worthless from a tactical standpoint. There is no purpose to controlling it and no reason to devote resources to doing so.
These are, essentially, planets/moons (which probably have defenses of their own), installations (also have defenses of their own, but not very impressive ones), jump nodes, convoys, and fighting ships. Against the Shivans you can eliminate planets/moons from your concerns.
Convoys are the most likely to suck up warship resources; they require powerful close escort, because they may not last long enough for reinforcements to arrive if they don't have such escort. FS' approach seems to be to provide a powerful enough escort that reinforcement will likely be unnecessary.
Installations will normally last long enough on their own merits to summon warship and fighter reinforcements to their defence. Nevertheless they also make logical canidates for leaving a warship or two nearby to protect.
Jump nodes are only of importance under specific sets of circumstances; i.e. you actually want to make use of it, or your enemies do. Since there's no way in or out of the system except for jump nodes, this is most of the time, if only to have a secure line of retreat. Nothing quite inspires confidence like the knowledge you can run away if it all goes to hell. Still, the guard forces would not be large, since a jump node really has no intrinsic value; like a door, its value lies in the ablity to provide access. If one side or the other can afford to devote ships to true blockade instead of securing other objectives, they are probably ascendant already.
Finally, the last object of value to be found in space warfare as practiced in FS are its implements themselves: warships. A destroyer, it goes without saying, is a high-value target: destroyers are what power projection is largely based on. They are thus worth defending: worth defending more then FS normally does. Most FS engagements are open-space battles against or in defense of warships that really have no good reason to be there unless they're hiding (or looking for other warships that are hiding). Rather then scattering warships about in ones and twos through empty space, gathering them together in a group would be safer in both the sense of their becoming harder to find (one spot vs. many) and more defensible. It would really be better, too, for the destroyer to join a one of the subgroups if all its escorts are commited to other tasks.
(The nebula may be a special case, since, probably many LY across, it may not have been possible to cross it in a single jump. A sweep through it would serve a purpose then.)