When I talk with sparker or say it without anything I usually pronounce it desu because that's how the girl from rozen maiden says it.
But usually if I am typing japanese (Kono Neko kawaii desu)
I usually pronounce it des. Someone said it is ruder but I've heard it said that way and sometimes it sounds better.

Like so des ka and so des
just sounds better that so desu ka? and so desu...


but that's just me