With steppers, you'll generally have high torque at lower speeds and vice versa. If you use a 1 start (.1" lead) you may not have enough torque at the speed you may need to cut.
Too coarse a lead and you lose the mechanical advantage that a fine pitch screw gives. That said, I've used 1" lead ACME on my router for 8 years and still able to resolve fine detail. I won't be doing wax jewelry molds but been happy with my results...
I'd say, under 2' get 2 start, and over, 5 start... Smaller motors, 2 start. Larger motors, 5 start...