hate to say it but I firmly believe that benching a scope to test PX error proves absolutely nothing whatsoever with regards real world shooting. I've generally found that it's the more expensive scopes that are likely to exhibit greater potential for PX error. I'm not using a fifty quid scope though!
The trick is to get to know what to look for when you've got an incorrect head position. Some scopes make this easy, on others it's a bit more difficult.