Adding a Bright Layer
I always like to add what I call a bright layer to the top of my Photoshop stack. This gives the ability to quickly turn on and off a highly adjusted layer to see the effects of gradient removal, any background color repair or to see if any of our selective sharpening or smoothing has had adverse effects. These things will show up much easier with the layer on.
To add the layer, simply go to the bottom of the layers palette and click on the add adjustment layer Icon….it's a little circle with half of it black….Choose the "levels" adjustment layer.
Now, simply run the right had slider up until it starts to completely wash out the center of the image. I rarely use darks or flats so this allows me to see how much of a hot spot in the center, and how much vignetting I need to fix with Russ Croman's Gradient Exterminator. It also allows me to see which areas I might want to crop.
This is sometimes very helpful to use after some sharpening to zoom in and see if you have gotten those nasty little halos from oversharpening. I start almost every image this way......install the layer, inspect for vignetting, color gradients..hot pixels etc. Then I turn the eyeball off and start my processing.