Burning Question - Do You Roll for NPCs in Combat?

An interesting question from Paizo's blog that pops up occasionally in my games but this happens a lot in war-based campaigns.

You have a scene where a large orc tribe is attacking a village. You have all of the orcs and various NPCs represented on the map. Do you roll for each and every orc and NPC, or do you, in the interest of saving time, just decide how many of each side dies each round?

For me if this battle is improvised, I would randomly pick off NPCs by probably rolling a d4 to see who get's removed. However, if planned, I would either pre-determine the order of which NPCs gets picked off in rounds or pre-roll a long list of numbers to give it an illusion of real-time combat happening before the PCs.

I'm very interested to know how you deal with this kind of situation.