One thing I have never understood about the borg is their use of time travel. They don't seem to be constrained by a "temporal prime directive" or whatever it is called. So if I were the borg, ever few decades or so, I'd send a ship back in time to give technology from the future to the past. This would also give decades worth of knowledge and exploration, insight into various battle techniques, and myriad other advantages to the borg of the past to exploit.
So why don't they use time travel to help themselves?