It depends on whether you are making 4 or 5 flights per outing versus 1 or 2. I use volt watch type indicators on my aerobatic ignition but not on the scale planes because the flying time is less.
It is pretty much personal preference and what you feel comfortable in doing. The volt watch is a good secondary reminder to turn off the ignition after flying if you are not using a servo operated kill and are depending on a throttle type kill setup.