Its quite interesting to see what everyone elses work stream is like.
It seems to me that most people discard the layers and save as .TIFF. As I said above, I keep all the RAW files and perform and save any modified/edited files as PSDs because it keeps the layer structure intact so that, should I ever want/need to go back to the image, I've got every single adjustment on their layers and I can see exactly what I've done previously.
Finally, I convert to jpeg for images being uploaded or sent off to anyone so that they're smaller (and I don't always want people to see the layer structure in the PSD!).