Chris, I've heard two different things regarding hip flexion in tall cleans. In Greg's book, he says no hip flexion, simply a slight lean back and a big shrug. However, my coach advocates for a small amount of hip flexion, as he says it better imitates the actual movement of the bar during the clean and discourages the slight pulling with your arms that can occur doing the tall clean flat-footed.
I have real problems with pulling with my arms during my lifts so I add the hip flexion in. I think you might develop faster movement under the bar without it, though.