The only prior contact with the federation was with the Hansen's as seen in Voyager. This would have introduced them to "us", but I doubt we would have been perceived as a threat, considering at the time the only people the Borg had assimilated were 3 civilians and one transport ship.
But they didn't just assimilate "3 civilians and one transport ship." -- the collective also assimilated everything the Hansens knew and whatever they could learn from their ship. While that still may not be enough to constitute a threat to them, it's a whole hell of a lot more information if they'd been taken by say the Klingons or the Kazon.
But that's assuming they had information on starfleet tech. I'd assume most of that would have been classified to starfleet personel. If the Borg assimilated me today, they'd know that the US Military has aircraft carriers and atomic bombs and the like, but they wouldnt know anything about the placement of airbases, missile installations or anything of any tactical value.
As far as I can remember, the Hansens were civilian scientists who studied exobiology. Outside of basic warp theory and weapons understanding, I doubt they knew much about the inner workings of Starfleet or federation technology.