Ian Williams wrote:
If your ukemi isn't instinctive, it's not much use..
Very good point.
As an aside - they only way for that to happen is lots and lots of practice. There is no fundamental difference between ukemi on any surface - if there is you should consider exactly what you are training for. What mats do provide is a relatively friendly way to get that practice time in and avoid both short term and long term injury.
Again - checking your ukemi on a hard surface is a good idea but I see no reason for consistently training there. Taking your training out of the dojo is always a useful exercise.