You don't need ditra but

I don't think that you really need to use the Ditra if you have a very stable surface and are installing a 12" x 12" tile. We had a similar situation with our concrete basement floor and didn't use Ditra at all. We installed a 12" x12" Italian made tile.

If you are going to use some of the larger tiles than you have to make extra sure that your surface is level. If you don't, that could cause a problems in the future. The self leveling material should do fine though. Here's an article from the Ceramic Tile Institute http://www.ctioa.org/reports/fr59.html for the larger formats 16" x 16" and larger.

In our kitchen, we used Ditra because we were very concerned about the stability of the wood floor. It has worked fine.