Rio: I like to force spawn away from the crates on Rio by guarding that ammo box (the one above with the Volus); it helps me as a frequent grenadier, too, because of the three grenades. When I'm running objectives to the extraction zone, I frequently hit both extraction-side boxes for the double grenades, and possible the singles in the center. I would say I hit the Box of Shame ammo box least of all on that map.
White: I like to force spawn down low, and so I'll frequently grab ammo at the single grenade box next to LZ, because on Plat it frequently has double-ammo, requiring less time humping the box. As I kill my way down to the basement, I like to hit the backside ammo box, and then the one near the ladder. I would say I hit all White ammo boxes almost equally based on my rotation through the map.
London: I like to force spawn away from my ammo box, which is the one behind the crate behind the Makos. I hit the LZ ammo box often for grenades, and the one next to the LZ-side staircase more than the one next to the spawn-side staircase (I tend to die a lot on Plat from phantom (Phantom?) Scion shots/grenade staggers at that spawn-side ammo box).
Ghost: I rarely hit the ammo box next to LZ or the one just outside of Building 7, though on a grenade run, I'll hit all of them.
Eh, this is getting lengthy. I'mma go play the game, er, I mean, acquire empirical data for this discussion.