also consider how tall the tank is...a shorter tank may look strange with a 1" sand bed, but a very tall tank will look just as odd since the sand bed will seem disproportional.
for example, my 37 gallon tank has a sand bed for the substrate, the tank is planted but the same principles remain. my 37 is 22 inches tall, and a 1" sand bed just looked to shallow. in the end, it all depends on how the tank looks after you add the sand, so there really is no set rule to how much sand your tank should have, planted or not.