I changed my default a while back and have found no reason to alter them - keep in mind though, that this is just a starting point and many photos will need different adjustments.
Both, shadows and highlights I have at 30/30/100. I have color at +30 and contrast at +15.
- Amount: is self-explanatory = strength of the effect
- Tonal Width: a low setting means that only the extremely dark (S) or bright (H) will be affected. 50% means that all tones below (S) or above (H) 128 will be affected.
- Radius: is the tricky one. Most experts (Deke, Bruce Fraser, Martin Evening, etc.) will tell you that this is a per/photo adjustment. Most will also tell you that a good starting point is to take a look at the WIDTH (in pixels) of the main area you wish to affect and set a radius that is HALF that OR LESS and then just move the slider around that point until you see "the sweet spot"... why the width and not the height I have no idea... but it does seem to work well. 100 is just a convenient # to start with as a default that works well if you don't want to adjust too much.
Color Correction: is useful for either adding saturation to the areas that are being brought into gamut by the other corrections, or desaturating. My +30 default tells you how often I think those areas need an extra bit.
Midtone Contrast: is a hidden gem of a setting that almost always makes the difference between a mediocre and a good adjustment. S&H tend to flatten the contrast. Not only does this setting help overcome that tendency, it is also one heck of a contrast adjustment that rivals (superceeds?) curves for this purpose AND it is much easier to get to know for the beginners than curves are.
There I go... being wordy agan :)