Some companies have qualifications that include your shopper rating. Where would they list this at? I haven't done any shops on these sites yet, so perhaps that is why I am not seeing any rating.
Not all companies give grades and tell you what grade they gave you. I generally ignore the grades anyway. More valuable to me is the feedback I get from those who edit my shops.
For shoppers who value their grades with great importance, the best way to maintain a good grade average is to do many, many, many shops. Then a few extreme grades, good or bad, will have little effect on your grade average.