I usually place all my finger on the fretboard. I do it mostly to get a better control over the pull-off from the pinky.

I would believe that someone with great finger independence and good strength with his pinky and ring finger could roll-it instead of placing all fingers on the fretboard.

The only advantage I see from rolling it is that it would be faster since you wouldn't have to wait for you previous run to end before starting the multi-finger pull-off.

Honestly, I rarely pull-off four finger runs. I either end up with 3 or if 4 notes are required, I would pull-off the first ones and slide to the last :)