A shower valve can certainly cause a leak downstairs. It may not be the valve, however. You may need to do some investigating to figure out the cause and then the solution.
If you suspect your shower valve is leaking and causing damage downstairs, instead of cutting the ceiling open, first remove the escutcheon plate covering the valve. You will have to remove the handle to pull the escutcheon away. You can use a flashlight to look into the wall and check the valve.
If the leak is coming from the wall, you will want to check for leaking water from the riser up to the shower arm. Remove the shower head and cap the shower arm, then turn the water on. This will pressurize the piping and if you have a leak here, you should see the water.
You might also look for a leak at the handle area and the possibility that water is running back into the wall.
Most of the ceiling leaks I see are the result of drains. Whether the connection from the shower drain to the pipe is bad, or that there is a loose or damaged fitting, the drain line is often the problem.
You may not be able to tell where the leak is coming from until you open the ceiling. Water will take the path of least resistance and it may be coming from 5 feet away, running down various obstacles in the way and finally dropping onto the ceiling where you see the stain.Can Over Tightening Toilet Tank Bolts Cause a Leak? » « Door Stop For a Hollow Door