Some keywords return the same results regardless of search location in the US. Some are entirely different depending on search location, and some are a hybrid. A score would be EXTREMELY helpful in determining whether a keyword is worth targeting. A localization score of zero could indicate that page one SERPs are virutally identical from all search locations within the US. A score of 100 would indicate that all page one results are different from region to region. All scores in between would indicate what percentage are constant vs variable.