Of course, some argue that leg curls are the best hamstring exercise. What people fail to realize, however, is that the hamstrings are not only responsible for knee flexation, but also for hip extension. Lately, a theory has come into vogue that grip spacing can influence the results produced from a given exercise. With bench pressing, the theory goes that a narrow grip works the inner part of the hamstring, and that a wide grip hits primarily the outer area of the hamstrings. This myth is slow to die, despite all the information that we have to the contrary.
Grip width has no real bearing on hamstring development.
Again, for best results, keep your heels up also. Also, have your spotter stand by your feet so that he can give you feedback on your hip elevation. Commonly, the spotter will stand behind the lifter's head where he can't even see the hips, and consequently, serves no purpose at all. Many trainees prefer the dumbbell bench for working the hams. I advise against this exercise, however, as most find it difficult to raise the hips when working with dumbbells due to balance problems.