I'm doing a rear extension with cable around my ankle from a machine. My weider poster shows standing straight for this with one hand on the wall for balance. A recent Oxygen magazine advises leaning forward onto the frame of the weight machine and doing this at an angle. It seemed to work it alot more this way, but which is right? Or is it optional? I suppose you get a higher/larger lift when you lean forward, working the glutes more. Is it just personal preference?

I lean forward. I feel like it works the glutes more and leaves the back out of the equation while giving you a larger range of motion. Also the machines that work the glutes (such as the butt blaster) are done leaning over so I would think it's the same.

I don't think it matters; do which ever angle you get the largest range of motion. However, be careful of your knees. Any exercise in which the weight is below the knee, can increase the chance of knee injury.