What is the benefit of a histogram in flash photography?

Have a Canon 300d with 550ex flash.

Usually use manual camera settings (M) or Av (aperture) setting when using
flash.

The resulting histgrams are mostly left weighted. I guess, this stands to
reason particularly if background is dark.

But this can also happen after setting the exposure (manually) for the
background, locking this setting and then recomposing and firing the flash.
Even setting the flash to 1-2 stops overexposure still gives a left-weighted
histogram and even the subject is not well lit.

What I am asking - is the histogram a good guide when using flash or is the
resulting image better left to post-manipulation?

regards

PeterH

Have a look he
http://www.sphoto.com/techinfo/histo...histograms.htm

Cheers,

Marius