Depends on the spy. From what I understand of them, the CSA spy, Belle Boyd gives a bonus to spotting for the entire region, so you just place her in the region you want more visability in, and she improves your spotting values. Pinkerton on the other hand, needs to move around the field to spot units, which, is considerably more dangerous. I've had him get caught a time or two, and he doesn't last long in battle.
