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

"It's a disaster waiting to happen, " Mr. Ulzheimer says.

WSJ: Cards Return to Campus

"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