Actually, there's a good reason not to time travel, then head to your destination. Since time moves at different rates depending on your speed, and since everything in the universe is moving at its own speed, time does not flow at the exact same rate at different places in the universe.
The Borg could've gone back three hundred twenty years in the Delta quadrant, but when they got to Earth, they might've found they'd only gone back two hundred seventy years. Or they'd overshot, and gone back to the 20th century (why not? Everyone else was). Since they had a very specific target, they'd only be able to get the precision they wanted by going back at their destination.
Going back in time elsewhere would also increase the likelihood that they'd change history unintentionally on their way.