But these services could be worth it, especially for those with no credit or who are looking to rebuild credit, says John Ulzheimer, president of consumer education at SmartCredit.com.

WSJ: The Rent May Help Your Credit

"You look no different than somebody with a big fat balance, " says John Ulzheimer, president of consumer education at SmartCredit.com, which sells credit-score monitoring services based on its own scoring formulas.

WSJ: Getting Going: Why the Latest Credit-Score Fixes Fall Short

"Margins on gasoline sales are already so razor-thin they can't offer you 50% off like a clothing store could, " says John Ulzheimer, president of consumer education for SmartCredit.com, a money-management and credit website.

WSJ: Gas Credit Cards Offer Few Rewards