When the Borg assimilate a planet, do they assimilate down to the last sentient being and assimilate the entire population, or do they keep a portion of the population around as breeding stock?
The evidence in this case seems to be contradictory. In Q Who when the Borg are introduced the only planet that the Enterprise gets to scan had had all of its sentient population removed:
A survey of the only Class M planet in the system reveals that while there was once a civilization there, it has been ripped away from the planet, "identical to what happened to the outposts along the Neutral Zone."
Indeed, all of the Federation colonies attacked by the Borg had no survivors:
When the Enterprise arrives at the edge of the Neutral Zone, they find that a number of outposts have been completely obliterated. There is no evidence of conventional weapons or attack. The Neutral Zone
"Captain's log, Stardate 43992.6. Admiral Hanson and Lieutenant Commander Shelby of Starfleet Tactical have arrived to review the disappearance of New Providence colony. No sign remains of the nine hundred inhabitants." The Best Of Both Worlds
However, in the Voyager episode Child's Play Icheb's race had been attacked by the Borg. Their once advanced civilization was decimated and reduced to a scattered agrarian society. Despite the fact that their planet sits at the entrance to a Borg transwarp conduit, when the Borg ravaged their civilization they clearly did not assimilate or kill all of the inhabitants. Indeed, the Brunali appear to get attacked quite frequently, and have taken extreme counter measures to try and make themselves unappealing for assimilation.
Voyager arrives at the Brunali homeworld. Seven is present on the bridge, helping to take scan data of the planet's surface. The planet is Class M, usually the mark of a very idyllic world. But there is a huge problem: a Borg transwarp conduit less than a light year away, which Seven detects. This explains Ensign Harry Kim's scan data of the population distribution: only scattered settlements, each with less than 10,000 people. This region of space is a Borg thoroughfare; the Brunali people live in constant, extreme danger.
Seven, Icheb, Tuvok and Captain Janeway beam down to the surface. The view is both breathtaking and foreboding; on the horizon is the ruin of an apparently technologically advanced city. Before them is a vast, deep canyon, as if the ground, and whatever was on it, had been literally scooped up and away.
Seven inquires as to how the Borg got [Icheb]. He sadly tells her: Since the Borg first attacked, they have been careful to hide any new technology, so that passing Borg cubes would pass the planet straight.
Seven asks the question the entire bridge crew gets in mind from that statement: Alone, on an unarmed transport? They respond that they are using the only weapon they have against the Borg, not having powerful starships like Voyager at their disposal: their genetic expertise.
Leucon angrily explains that every time they begin to rebuild, to make progress, the Borg come and take it away. ... "You have no right to interfere!" Leucon shouts. "We're trying to save our civilization!" Yifay pipes in. Janeway glares at them icily. Seven accusingly tells them that they are trying to do so by depriving Icheb of his future. "If we don't stop the Borg, the Brunali have no future!"
The only determination I can come to when considering this evidence is that in some cases the Borg do go out of their way to not completely eradicate a species, most likely when that race shows the potential for additional advancement later. In the case of the Federation colonies and outposts that were destroyed in The Next Generation, the Borg likely (correctly) surmised that these small settlements were not the end of the their species, and as such there was no reason to not assimilate every last person there.