I've never attended a MapUp, but to be truthful, its usually just a case of progressing down a street. If street hasn't been cleaned up before/recently, which is obvious by the markers mainly being on the street instead of at the front doors of the businesses, you will often find that pretty well every business needs to be cleaned.
Even those claimed in G+ are usually in need of repair, as most owners go with the location off the marker on the street, G+ doesn't send over address information very well to Maps, and owners use inappropriate names and categories all the time.
Most, however, are not claimed and thus need to be cleaned also.