Levels then Curves, or Curves then Levels as the first steps. I know it is going to be a subjective answer, but after opinions and justification anyway.
All arguments will be settled with pistols at 20 paces
Trevor I assume you are referring to astro image processing.
First you need to look at the histogram and see if there's any bias in any of the channels.
That means that the curves all need to start to curve at the same point on the histogram.
Do a levels adjustment and get them all to coincide and then proceed to do your stretching using curves.
Always keep an eye on the histogram and remove any bias between each channel which leads to colour casts and be careful not to clip the histogram at either end.
for CCD i use curves followed by quick color balance once object is recognizable to some degree, off to Deconvolution, then ...... lots of choices. Digital deveopment, more curves, Shadow/ Highlights, or import into another program (maxim, lightroom, WIN4AIP etc) lots of tweaking and not just a linear process, details from earlier versions can be reblended as required etc etc etc.
