I used to get all excited and upon opening the image up in Photoshop, immediately start processing skin.
Nowadays, I want to see the general idea of the finished product before I commit hours to an image. So what I do now is play with all my layer adjustments before I process anything else. This includes my B&W conversions, levels, curves, saturation/desaturation, gradient maps, exposure, etc... before I go into dodging and burning and skin smoothing techniques.
The reasoning is simple: How do I know A) if this image has "potential" and B) how the final picture will (generally) look without applying these major layer adjustments?
Lots of times, I "test" images out in PS before I try actually editing it. "Testing" for me might only take 2-3 minutes. Liquifying or working some particularly nasty challenge that might make or break the image. Only if I know I can overcome the challenge, will I be willing to edit the image all the way. Sometimes even if I can, it's not worth the time and I'll opt for an easier image. But usually I'm up for the challenge even if it takes a lot of time.
These quick layer adjustments only take 5% of my time in PS, yet they will reflect 95% of the total changes in the image. Curves, color balance, exposure, levels, gradient maps, B&W conversions, sat/desat etc. are BIG items. Ironically, the other 95% of my time in Photoshop is spent on minute details that change only 5% of the image. Sure, this makes no sense on a cost-benefit analysis... but that 95% of the time I spend doing D&B and correcting blemishes is the difference between a refined/polished image and a cracker-jack snapshot. It's what makes a L U C I M A image what it is.
But by retouching "backwards" i.e. starting out with the general idea of the final look of the image, you can get a better idea of how to retouch the image. Sometimes little details in the skin tones won't appear until you've converted it into a B&W image. That's why retouching backwards is so important... it changes what you'll retouch and how you retouch.
Try it, it should not only speed up your workflow but lessen the amount of unnecessary brush strokes and adjustments!