In "First Contact" the Borg travel to the Earth's past to change history. Have the Borg ever made use of time travel as a weapon, much like (say) the Krenim in the Star Trek Voyage story "Year of Hell", or considered time travel to make assimilation easier.
sounds cool - they should make a movie about that
Star Trek: First Contact
During an attack against Earth, the sensors detect that the Borg sphere is creating a temporal vortex; the crew all watch the viewscreen as the Borg vessel disappears through the vortex and Earth dramatically changes. Seeing that it is now populated entirely by Borg drones and has a toxic atmosphere, they determine that history has been changed, having been protected from the changes themselves by the wake of the temporal vortex. As the vortex collapses, Captain Picard orders Lieutenant Hawk not to alter course and have the Enterprise follow the Borg into the past – to repair whatever damage they've done.
(note: the film was released in 1996, and the Year of Hell in Voyager was broadcast in 1997)