I've never seen or heard anywhere that Voyager was about to be cancelled. At least not after the three first seasons.
The ratings were falling but it was the same for DS9 and there were no talk about any cancelling of that show either.
As for the rating things, the ratings got even worse in season 4-7 except for a short increase directly after Seven joined.
http://img.photobucket.com/albums/v433/Lynx677/Star_TrekVOY27157.gif
So using the Borg didn't help that much.
Maybe they should have put down more efforts in coming up with good stories and really using the excellent characters and the excellent premise the show did have.