If you're in a pinch where you need a hole to access the box, and don't have any sort of bushings here's a trick I use:
Figure out where your box is gonna in relation to your fixture. Drill a hole in the fixture that marks the center of the box. Use tin-snips to cut an "X" in the in the fixture, the hole you drilled should be at the center of the "X". Then fold back each of the triangles formed by the "X", and press them flat with your kleins or a hand-seamer. It leaves you with a square opening in the fixture, and the rounded edges won't knick your wires.