I just switched from 100% to 90%. I'll probably stick with 100% for my K100D photos... but with the K20D, it's unlikely that I'll have a display device any time soon that'll be able to show the full 14.6mp image at once, and viewing at 100% with 90% and 100% side by side, I sure can't tell the difference. But I can with file size - my test photo was 7.65 megs at 100% and 4.06 megs at 90%. That will save me a lot of room in converted jpgs!
If I am going to do a print, I'll probably save to PNG (or compressed TIFF if I absolutely have to - it makes me insane that in the year 2008, the primitive TIFF is still in use at all), so that'll take care of "top quality" images.
80% does sound low. I can definitely see the difference between 80% and 90% when saving jpgs with other programs, I doubt Lightroom is doing any magic that is allowing it to compress further with no visible artifacts. For what it's worth, I generally save my resizes (both on my PC and the ones automatically done on my web gallery) at 85% quality.