Do I have to use Kerdi tape with Ditra membrane?
"Schluter®-KERDI is a pliable sheet-applied waterproofing membrane and vapor-retarder designed for the direct application of tile. KERDI is ideal for use in tiled showers, bathtub surrounds, residential steam showers, and other tile applications in wet areas." Schluter Kerdi

I would consider Kerdi tape or membrane to be overkill anywhere outside a shower. Ditra is indeed waterproof, but its primary purpose is to serve as an uncoupling membrane, which prevents movements in the subfloor from cracking tiles. The tile and grout are intended to create a water resistant barrier sufficient for keeping drips and spilled water on the surface long enough to be addressed with a mop or towel. If you want to be thorough with your bathroom install, make sure you seal your grout or invest a little more in a urethane grout like CBP Fusion Pro.