People who write "I should of" are viewed as stupid, and for good reason. You get the impression that that is a mistake that only a (very thick) native English speaker would make; a foreigner would never write it. But I've never known anyone actually openly state that they truly think that "I should of" is proper English. Do you really think that they would teach their children that "I should of" is proper English? Surely it wouldn't be that long before someone pointed out, following proper consideration of the three words as individual words, that "I should of" isn't proper English at all and that what you should say is "I should have"?

Consider me having said the same thing for "your" used as the abbreviation of "you are" and stuff.