I believe that when you are a beginner, you should practice on fairly soft tatame because it will prevent injury due to lack of experience with Ukemi. However once you become adeped in the ways of Ukemi you should train on hard tatame or with no tatame at all so that you have more of a reason to practice proper Ukemi. I have heard many stories of people who learned Ukemi very well because if they didn't, they would be in pain for several days. So, I beleve that the more experienced you are in Ukemi, the harder the falling surface should be.
In Aiki Spirit,