.dds can be saved into lossless format.
All the DXT ones are lossy as well as the seldom used 3Dc.
You usually want to save the file as 8.8.8 RGB or 8.8.8.8 ARGB when working on the file and once finished, pack it into DXT format.
Grayscale images can be easily saved with 8L, and just extracting the alpha channel from image save it as 8A.