How critical are proper margins on a resume?

I have been told repeatedly by career centers various pieces of advice about resumes and one of the things that often shows up with little to no explanation is to have 1" margins and use 12 point Times New Roman font. I get wanting to standardize the format of resumes so that no one is able to cram more information than anyone else, but how critical are proper margins? As I go to update my resume, is there a reason I should hold myself to that especially considering I see a large number or resumes that don't follow this format.