For example, if my players ambush a pair of Stormtroopers, they'll probably each be Rival level, since it's more of an individual fight.
Just curious, why would you upgrade them to Rival and increase their overall difficulty instead of leaving them as Minions?
My group and I played a lot of WH40k RPG before switching to Edge, and so we generally run grittier, more down-to-earth campaigns. Specifically, "NPCs are people too". This means they (generally) want to survive, so where retreat or surrender is a viable option they'll take it, but it also means they fight smart and are proactive, and won't just passively wait for the PCs to do something.
Joker's comment reminds me of 7th Sea where today's Villain-level NPC is tomorrow's Henchman-level NPC in a different scenario. It depends on how the NPC is used and what level of importance he is at the time he is encountered. Even an individual Brute (or Minion) can be important, and a pain to deal with, during specific scenarios!
RLogue hit it right on the head here. Those two Stormtroopers patrolling the back streets near the spaceport while the PCs are trying to free the slaves are incredibly important, plot-wise. They are struggling to fight off their attackers, alert their commanders, and desperately flee the angry mob of freed slaves drawn by the sounds of the fighting. The results of this encounter, and the way they are achieved, will have consequences for the rest of the session and probably into the next one or two that follow as well.
They'll be Rivals (and I track Strain for Rivals) so that I have more versatility in modeling their response to the ambush and can describe the details of the encounter with more granularity. Such a plot-critical fight should be the climax of the session, and I'll stretch their Skills, Talents, and Equipment to the max to challenge the players appropriately. If either of them survive (or die in a spectacular way) they'll probably get names and at least a defining quirk or two, and I'll keep them in the back of my brain (or in my notebook) in case I ever need another Stormtrooper with personality, because the players will certainly remember them.
On the other hand, once the governor's been alerted, and the PCs are arming freedbeings and leading them to capture the hangars before the garrison can scramble its TIE Fighters, one or two Stormtroopers don't matter a whole lot. They're still quite lethal, of course, but with a larger-scale encounter there's no need to track the individuals who make up a crowd, as the upgrades to their Ranged - Heavy from being a Minion Group gets the point across well enough. When they're torn limb-from-limb by bellowing Wookiees seconds after the TIEs launch, the PC's would rather know whether or not they've convinced the Rebel gunrunner to dump its cargo and help them escape than hear what TK-422 whispers with his dying breath.
I'll even change a NPC's status mid-encounter, if circumstances dictate. The last survivor being interrogated for information or persuaded to defect is a lot more important plot-wise than the seven squadbodies who aren't, and so I'll usually kick him (or her) up to Rival just so I can give the interaction greater detail.