Actually for most panorama shots, I have just been erring on the side of caution and shot with a huge overlap - you can just pick which of the shots to use when you do the stitching. If you use a tripod you can adjust more precisely for a 30% overlap, but like you say the overlap is adjustable in post processing. I sold this geese photo to a hunting magazine: it is stitched with pandora from five shots, but I actually took eight and picked the five with the best overlap later.
http://dl.dropbox.com/u/10485935/MHHH7659.jpg

One thing, if you take photos at 18mm for building panoramas, your perspective gets a bit distorted and stitching becomes more difficult. I find it easier to zoom in a bit and stitch more shots together to get the scene you want.