To answer your original question, it really depends on the severity of the defects. I would prefer an Unc note with a stain to a VG note (lots of creases). Centering, corners, edges are important, but mostly when you are looking at Gem notes.
Also, stains and dirt can sometimes be removed. I don't advocate altering any original notes but removing stains is a legitimate form of conservation, provided it can be done without causing additional damage. Some types of stains (eg: foxing, mold, etc) in fact should be removed to prevent further deterioration. It might be worth buying such a note at a discount if you can have it restored with an experienced conservator.