Ive also been using picasa as a quick touch up/viewer recently, and I also have my stills mounted inverted. For bulk rotating ive been using a second program: XNView (at www.xnview.com). It can do lossless jpeg transforms - it reorganises the data in the file instead of uncompressing the picture before doing stuff to it and then recompressing it. So basically you can do a 180 rotate without loosing any quality at all.
Itll be no benefit if you are shooting in raw mode, but it might be worth a look...
