Target bigger MSC's as they have the bigger clients - chains that are more likely to be shopped and have locations in every little town in the U.S. Most of these MSC's are on the "15 Most Discussed" list on this site.
Rural areas can be challenging: As a part of being a rural area, there simply are not that many locations that could be shopped - much less that are shopped. This limits possibilities. However, there can also be some advantages: There are fewer shoppers, too. It is not uncommon for MSC's to have a terrible time getting their shops filled in those areas. Once you learn what your shopper competition is in that area, you may be able to get those shops in your general area with better bonuses than what folks in more urban areas can get. You also might need to travel more in order to shop.... (It's hard to be a mystery shopper in an area where there are not any stores.)
I travel to rural areas periodically on routes to pick up heavily bonused shops that are difficult to fill. It makes for some very profitable days.
Hard work builds character and homework is good for your soul.