Better overall hamstring development is important for both symmetry of the thighs and protection against injury. In addition to quad developers like the squat and leg press, you should also be doing hamstring-specific movements such as stiff-leg deadlifts and leg curls.
Research has shown lying leg curls do not stress the hamstring in identical fashion as stiff-leg deadlifts: Leg curls activate the lower and outer portion (biceps femoris), while stiff-leg deadlifts place greater emphasis on the upper medial portion.
For complete hamstring development, combine both leg curls and stiff-leg deadlifts. If the equipment is available, progress from lying leg curls to seated leg curls after six weeks or so. This will increase the degree of stretch on the hamstrings as long as you make a point to sit up straight. To reduce stress on the lower back, perform stiff leg lifts on a hyperextension bench.