Skinning Tweaking

Very rarely....if ever only to reduce background brightness...to reduce eye-strain...;)

Never.

i do- i use windowblinds to texture the skin and i recolor them often.
I think its one of the very best functions WB has to offer.
I have made some changes in skinstudio but only to the diamond skin or win8 metro-to know how certain parts work ( all in private use of course )
to the fonts i like Arial or Verdana so yeah i changed the font on some skins but only to be readable without glasses.

I change all of Vstylers skins to super gaudy colors via WB then send him screen-shots.  I imagine his brain bleeds and his heart dies a little every time.  Sometime, when I'm feeling really low, I'll add a zebra-stripe textrue to it just to lift my spirits. 

Seriously, I do use WB to hue shift some skins; mostly to the green side.  I like green.

On the first question: Rarely if ever.  If a WB has a fair amount of "companion" elements (docks, WS theme, IP, DX, etc.) changing the color of the blind throws everything else off.

Usually the skinner has a better eye for color than I and a general look they were shooting for. Who am I to muck that up?

I do apply a texture once in a while, the change can be quite dramatic.

On the second: Not in a million years. I get my kicks breaking RnD's DX stuff, so no need to mangle any WindowBlinds.

I change fonts usually size for eye relief in some cases, I change colors to get better contrast in some cases, I prefer Blue as my Hot tracking color so if that is not the case i change it. I usually change the title bar color and gradients and Borders and scroll bar colors as many skinners leave these as one solid color or default colors. here is an example I changed a few in the new diamond star skin

I also change the Branding in all skins that do not have a images and are only colored.

Some times background or font color is changed as the color is light on light or dark on dark giving very little contrast as to be seen well

I have never changed anything in a WindowBlind.

I don't but do look at how my skins will recolor and what the textures will do.

Haven't tried to......... yet.

Yep.  Also use WBConfig to change system colors.  Mostly for visibility/legibility on all the above.  I frequently shift a skin color in WBConfig to go with a wallpaper, sometimes add a texture for the same reason.

Also to a fair amount of tweaking in SKS on the vertical taskbars.  They're a little contrary, you kind of either need to set their width to look good with the skin (much easier, just drag it), or tweak the skin for the width you like to use (much more "work").  What can I say, I like them skinny.

All great features as far as I'm concerned. Flexibility.