I guess it depends what level you are at. Some may mark you down but it depends what class you are in. I think at low level you should be ok. You may get a picky judge and if its tight between placings they could passibly mark down but i wouldnt let it put you off.
It depends entirely on how noticeable the scar is. Most are coverable with makeup. However, showing is about being as close to perfection as possible, and if two animals are otherwise equally matched (in conformation, manners, movements, paces, temperment, breed standard etc) then the unblemished horse should be placed above the horse with scars.
I think its stupid that they're marked down on scars. If a horse is living naturally its bound to get scrapes and scratches.. Neo would be no good, he's only 6 and is full of white marks in his dark bay coat!
I think it depends too. i'd expect to see a dainty show hack blemish free but horses such as working hunters can have a certain degree of "honest scars" as they are supposed to be examples of working animals.
I used to work on an arab stud and they had a gelding that had won at the nationals against stallions (geldings winning over stallions is unusual at top level arab showing) He had a whopping great hairless scar on his chest/shoulder from getting caught on a gate bolt. The owners opinion (and obviously the judges) was that even though a lot of us see arabs as airy fairy show animals they are actually bred as war steeds so scars that dont hinder movement etc shouldnt affect the horse.
Scars will only come into effect depending on the type that they are. For example a scar that is just a clean line, slice type of scar will have no impact on results.
However scars that cause thickening, say on the leg, may well effect results depending on it positioning and severity. White hairs as result of an accident may not be directly put down however they will draw the judges eye to said area and make it obv that there has previously been an injury and any slight imperfection may be more noticeable. For such reason i would always cover white marks or scars on legs.