I pretty much use your method, but I have some slight changes.
1. I use the LASSO tool to select the teeth (or whites of eyes.) IMHO this is MUCH quicker than using the Magic Wand and varying the settings. I usually use a feather of about 2 pixels for a high res image, less for a low res image. (Obviously, I zoom in on the teeth to do this.)
2. I put the selection on a seperate layer (Ctrl-J, PC or Cmd-J, Mac.)

3. Create a Hue & Saturation adjustment layer over the selected teeth.
4. Place the cursor over the line separating the layer with the teeth and the adjustment layer, hold down the Alt key (PC) or the Option Key (Mac.) The cursor will change to a circle with a looped arrow. When you see this, click the mouse once.
You will see the icons for the adjustment layer indent to the right. This "restricts" the adjustment layer to ONLY the layer immediately below (the teeth or eyes.) (Normally adjustment layers affect ALL layers beneath them.)
5. Hide the "marching ants" by holding down the Ctrl key (PC) or the Cmd key (Mac) and hittuing the "H" key. This makes it easier to see the effect of whittening. the teeth.
6. Zoom out to a normal view. (It is hard to judge the color tof teeth without looking at the whole face.)
7. Reduce the Saturation by 10 and increase the Lightness until the teeeth are too white and then back off. I usually place the cursor in the box holding the numbers and use the up and down arrow keys. This changes the numbers by one. If you hold down the shift key it changes the numbers by 10.
6. After I'm sure the teeth or eyes look OK, I merge the three layers. I don't do this until I print the image, so I can correct any suprises. (If I'm not going to print the image, I probably wouldn't bother whitening the teeth or eyes.)

George Burke