The roto tiller idea sounds good, but it would need to be repeated at least once every year.
As to your original question, yes, 6 inches is plenty, but only if the space remains clear and open. It you have trees around that drop their leaves in the fall, they can quickly get caught in the gap and eventually fill it and eliminate the air gap.
Last year, I discovered the problem with some large pots. There were more tree roots than plant roots. I had to empty the pots and remove the roots from the soil. Once done, I put a small piece of concrete down, set the pot on that and refilled it. No more root problem and the pot is still able to drain freely.
