Vaul is right, take WW2 for example. The speed and size of a WW2 DD coupled with its limited radar/sonar range makes a quasi-decent comparison to a Starship in a hEXX (little hexx, isnt that right?) Anyways, if a given hex belongs to an empire, there are going to be patrols of some sort, even if not actual Naval warships, perhaps police ships, long-range fighters, etc.
Convoys will have escorts and Q-ships, etc. Not to mention listening posts, asteroid bases, etc.
While Im sure there are vast reaches of space with absolutely NOTHING in them just as Hexx (big hexx?) postulates, it would be easy to tow a decent sized asteroid into a hex, or drop a mobile base, listening post, whatever.
He is right however, Id make hexxes with "nothing in them" no higher a DV than 5? Unless its an "empty" hexx within the core of an empire, where there is going to be far more Military and police traffic on a constant basis.
Line of supply is just an indicator that Starships and transports can expect to freely (within reason) travel thru these hexes without being subjected to enemy attack. The odd pirate not withstanding, your not going to expect to find a C7 3 hexes from Earth unless things have REALLY, REALLY gone bad. (Or Fluf made the map)
If its a neutral hexx, er, hex, that simply means it is not patroled and therefore it is not safe to send merchant traffic thru, and Starships have to be ready for combat at any time while traveling thru these hexes.
Sorry Supreme Imperial Dicktater Hexxipoo, your not going to get LoS knocked out that easily. We KNOW youd feel more comfortable deepstriking in the Federation rear where there wont be anyone looking for you, but thats not gonna happen.