If the Borg found Earth in 20 century or early 21st century, such as 2015, would they assimilate humanity? If they thought Earth is too primitive to their use, would they just leave Earth alone for later, kill everyone on it, or forget about Earth?
The Borg would most likely skip over earth for assimilation based on our tech, and humans physical anatomy.
The Borg have been shown to skip over races with Warp tech so a pre warp tech wont pique their radar.
Species the Borg found unremarkable would be deemed unworthy of assimilation. As of 2374 the Borg considered the Kazon beneath their notice, and by 2376 they only took interest in the Brunali if they detected sufficiently relevant technology.
Of the Kazon.
"Their biological and technological distinctiveness was unremarkable; they were unworthy of assimilation." When Neelix commented on this heretofore unknown discriminating nature of the Borg, she replied: "Why assimilate a species that would detract from perfection?"
memory alpha (VOY: "Mortal Coil", "Child's Play")
I expect they would still assimilate a 20th or 21st century Earth. However, as pointed out by @Himarm in the comments below, I base this answer on the premise that they would assimilate us if they understood something about the general personality traits of our species. Whilst we are pre-warp, technology is only one consideration. Similarly, biologically the Borg have described us as:
Physiology inefficient, below average cranial capacity, minimal redundant systems, limited regenerative abilities. (VOY: Dark Frontier)
Aside from our biology and technology, the Borg are also interested in our abilities and humans have been noted several times by the Borg that we are unusually resilient and persistent creatures not to mention our curiosity. Adding sch traits would only add to their perfection. So, assuming that they were able to observe humanity and come to judgements about the traits of our species which they have evaluated as desirable, based on these human traits I think the Borg would probably assimilate a 21st century Earth.