NEVER MAKE CONTACT WITH THE BOTTOM
This rule is recited even more when people talk about bottoms that have living coral that would be disrupted or killed if contact is made with it. While best practice definitely would be to look and not touch, however that is not always possible. There are certain times when this is not possible, such as when there are strong currents or wildlife in the area that may be kicked by a neutrally buoyant diver. It is also far better to gently rest on the bottom when learning to use a new camera than to crash into coral or other marine life attempting to adjust settings and line up a shot.
The best rule of thumb if you are unsure whether the location you are in allows you to make contact with the bottom is to ask. Each area is unique in its marine flora and fauna and the local dive operations can provide the best instruction.
So while the best intentions are behind this rule, it is far better to adopt the stance of low/no impact diving over never make contact.