To answer a question like this, just take a look of the function of the muscle in question. In this case, the hams are knee flexors AND hip extensors. Now a lot of people will advise that the hams are heavily involved in rock-bottom squats (and I'm not necessarily disputing this) because in the bottom position of a squat, you're in a high degree of hip flexion, which of course places a lot of load on the hip extensor (namely the hams).
However, a more sophisticated observer will also notice that, in the bottom position of a rock-bottom squat, the knees are also in a high degree of flexion. So what does that mean?
Very simply this: a good deal of the tension that the hams experience from the hip flexion is countered by the relaxation of the hams due to the flexed knee.
So that's the ideal positional configuration to place the greatest possible tension on the hams? A large degree of hip flexion combined with a relatively extended knee. That leans stiff-leg deadlifts, RDL's, good-mornings, back extensions, pull-throughs, reverse hypers, etc.
Now don't miss my point... the hams are certainly stimulated in almost any type of squat. Just thought I'd add a bit of biomechanical perspective to the discussion...