it's always safer too underexpose and save your highlights from oblivion, but how much underexposure is safe if i want to rescue that information with a power window later? Anyone had any experience with this situation.

I don't think a specific answer to the question is possible. It depends very much on the subject matter and personal preference. The problem with trying to fish detail out of the shadows in 8 bit wide data is that there just aren't enough levels. The histogram breaks into a "picket fence" and the image becomes "posterized". The amount of this which is tolerable depends on the individual. Experiment!

not much...I always underexpose, but usually I use zebras at 90 and turn them til they just disappear from my subject. Maybe a little more down if I can save some highlights by doing so as well. Thumb wheel on the XL1s clicks, I don't go more than 2 clicks under where the zebras disappear. Footage I intend to project/TV I tend to run slightly darker than stuff I intend to be seen on computer.