Al Pascual, senior analyst for security, risk and fraud at Javelin Strategy & Research, said in an email that tools like BillGuard could cut down on the virtual legwork needed to monitor multiple accounts. But as with other tools that aggregate financial accounts, you must be comfortable sharing your account passwords to register cards with BillGuard.

The company says it encrypts personal information and uses Yodlee, which works with many big banks, to verify credentials; BillGuard doesn’t store them. The app obtains “read only” access to your transaction descriptions and amounts; it cannot move money into or out of the accounts.

Still, Paul Stephens, director of policy and advocacy at the nonprofit Privacy Rights Clearinghouse, which advises consumers about fraud and identity theft, counsels caution. Given the prevalence of data breaches, you must weigh any benefit from using a service like BillGuard against the potential risk of sharing login credentials with an additional business: “The fewer entities that have your information, in my view, the safer you are,” he said.

Here are some questions about using BillGuard:

■ What if I’m uncomfortable sharing my login information?

Mr. Pascual of Javelin suggests that you use tools offered by your own bank and credit card companies, like text or email alerts that notify you about transactions. That can help you track charges without logging on to each account — although you should still do that periodically. It’s crucial to monitor your debit card charges, Mr. Stephens said, since unlike credit cards, debit transactions deduct cash directly from your bank account.

The Privacy Rights Clearinghouse recommends that you limit the use of debit cards and use credit cards when possible, because of the generally stronger consumer protections available with credit cards.

■ Is there a fee for using BillGuard?

The app is free (it once charged to register more than two cards but no longer does so). BillGuard aims eventually to earn fees from merchants for its dispute resolution and related services, Mr. Samid said.

■ What if I want to complain about a “gray” charge on my account?

If the merchant won’t remove the charge, you can contact your bank or credit card company directly. You can also lodge a complaint with the Consumer Financial Protection Bureau, which has taken action against several credit card companies for using deceptive practices to enroll consumers in products they didn’t want.