The rule of thumb I would use is this:
"Is the ship you're killing unique? Ie, is this the hundredth Guristas Obliterator you've destroyed today? Or did you fight and kill Sami Okuda of the Caldari Navy?"
Now, obviously, there's a grey area. But essentially, was the event unique in some way? For instance, I've been on tons of Friday night roams in my career, and while many of them were similar, no two roams were identical. The timing, the targets, the outcome, the fleet comp.
So, they were all unique, shooting and destroying target ships hurt the targets... they're not some faceless NPC, who spawned in said ship when you warped in from an acceleration gate. A faceless NPC who will behave the same way the first time you fight then as they will the hundredth, barring expansions or changes by CCP.