Curves in Photoshop

I am new at digital output. One of the things that confuses me is the matter of the use of curves in Photoshop. Presently I am using the layers palette and using the color sampling tool in the threshold window to locate and set my shadow and highlight points in the image. I then close the threshold layers and convert the color image to black and white using lab color and greyscale followed by another layer in which I select either multiply or screen (depending on the image) and adjust the opacity to the best image I can achieve. I then open the curves window and sample my previously located shadow and highlight points in the image and this locates these on the curve. This gives me a purely linear curve without either a toe or shoulder as one would normally encounter with film or paper profiles. Is this the best way to do this or would I be advised to build a curve incorporating a toe and shoulder? If so can one curve be used on all images? What are your practices? Thanks in advance for your experience in this regard.

Hi Don

I by no means consider myself an expert of PS.
Practice has shown me a few things that are strange but apply at my Lab.

Working from digital capture is quite different than working from scans.
therefore I approach contrastdensity or *curves* differently.
Now I always open the image in the printing profile required.
For digital capture and scans I go to curves first.
digital capture I wil try a Auto and see how the image responds, if I like what I see I then will modify the curve shape to what I think the final image will look like.
scan images I will try a Manual adjustment, basically dividing the line in threes, moving the mid highlights up a bit bringing the mid shadows to a point I like an using the mid point to slightly adjust the density.
After this curve adjustment I will go to levels and do a minor adjustment.
When I first started I was afraid of adjusting curves and worked with levels a lot. Then I started using both on the same image.
Lateley I have found this can be dangerous and will result with posterization or banding*not sure why*
I find the curves to be easy to use and with the lambda I can adjust contrast/density/colour on top of what is being done in PS.

If you are new to this I would suggest the method of outflanking described by M Smith. Just go overboard and see the results and then back off to what is appropriate for your printing needs.
have fun Don, I know I am.
Just purchased CS3 and capture one and am excited to try them and see how Cs3 helps our workflow.