Bob,
You must determine the goal of the exercise. If you are targeting the hamstrings, you must maintain a flat back. This makes the movement exclusively hip extension, of which the hamstrings are the prime movers (except for the short head of the biceps femoris - as it only crosses the knee, it is best hit by knee flexion movements - but the other hamstring muscles cross the hip).
If you are targeting the spinal erectors / "low back", the that is the joint(s) that must move - the lumbar spine. However, given the loads imvolved and the moment arm of the muscles and the resistance (and other mechanical factors), the "SDL" is less than ideal for spinal erector work. Lots of risk for reward that could be had with "safer" and possibly more effective exercises.
If you allow the back to round, you shift some of the load from the hamstrings, to the spinal erectors. The spinal erectors are active isometrically because of load placement, but not concentrically/eccentrically unless the lumbar spine moves.
Do'nt know about you, but I have a different strength ration in my spinal erectors and hamstrings. Not sure if I would want a load appropriate for one group thrown onto another.
Flat back is generally safer.
On another note, spinal disc integrity decreases by 50% when the spine is forward flexed. This is made worse if you are loaded - even worse if seated - and reallllly bad if you also rotate (somewhere in the neighborhood of 85% loss).
This is why most people injure their back, not with a large load correctly lifted, but a light load lifted suddenly (larger force on lumbar spine - f=m*a) with forward bend and rotation.