The op amp is oblivious to the actual power supply, so this part can be used on a single supply from 5V to 34V. Yes you are right in saying that this is the same as using the part as +/- 6V. For decoupling, put a cap from 12V to 0V and one from 6V to 0V. Do not put one from 12V to 6V, or this will couple noise from the 12V into the reference. Ultimately this depends on what your input signal looks like. Does this go below 0V? If so, you need a power supply that goes below 0V so the op amp can recognise signals lower than 0V too.