The only big problem comes, as Mike stated, when they (the radio shop) start issuing Radio IDs willy-nilly and that's when it becomes a nightmare.
You'll find this to be the case in most counties where they're Tier 1 or Tier 2 MARCS customers; MARCS issues IDs from the xx9xxxx block on a "next available" basis, meaning Department X sends in a list of 10 radios which are issued the next 10 available IDs. Department Y sends in a list of two radios, and they get the next two available IDs. Then Agency X comes in again with five more radios, and gets the next five IDs. And so on. . . .
In some places it can really be mixed up depending on how the list was sent in; if it was a hodgepodge listing sent in on a countywide basis, all bets are off.
The annoying thing is that the rest of the county range (xx00000 through xx89999) is available, and a Tier 1 county could put together an ID plan if they wished, assigning specific blocks to specific agencies. Even if the IDs were assigned randomly in each block it would make life easier for everyone (for example when an emergency banner goes off, one could at least look at the ID and know which agency it is).