Susan, you need to distinguish between straightening and levelling a
panorama:
Straightning is done blind by analysing the image parameters. It
works best for partial panoramas consisting of rows of images, but
less well for full 360x180 panoramas. It might give you a nice
straight row of images, but since the image content is not taken into
account, the levelling of the actual panorama can only be as good as
the levelling of the panohead (if one was used).
Levelling, OTOH, works with data input manually to identify features
in the images that would be expected to be vertical or horizontal as
they appear in the output panorama in the chosen projection. The
features are identified using additional special control points. See
http://www.johnhpanos.com/levtut.htm . This works well and is
independent of the levelling of the panohead.
For manual straightening/levelling, see
http://www.ptgui.com/man/straighten.html
John