"May I help the next customer?"

You finally make it to the front of the line and dump your clothes on the checkout counter.

As the scanner displays the price of each item, you start having second thoughts about whether you truly need two new pairs of jeans ... and shoes.

That's when the salesperson hits you with the dreaded question: "Would you like to open a store card today? You'll save 20% on your order!"

What's a young shopper to do?

Know yourself

If you're prone to shopping binges or are already struggling with credit card debt, signing up for a store account is probably a bad idea.

Interest rates on store cards are notoriously steep and can "quickly devour any point of sale savings you can get by using the card," says Eric Dostal, a New York-based certified financial planner.

Related: Uber's new credit card targets Millennials

Opening any type of new credit card also involves a hard pull on your credit, which can send your score down. The drop is usually temporary, but you should be careful about how many hard pulls you allow, especially if you're getting ready to buy a house or a car, adds Dostal.

If you pay off your credit cards every month and you're a responsible spender, opening a store card could help you build credit and save money on purchases.

Still, the credit line on store cards is usually low, which means you won't build as much credit as you would with a traditional Visa or Mastercard, explains Hank Mulvihill, a Texas-based CFP.

Ask the right questions

Before signing up, you'll want to inquire about any annual membership fees. If you only shop at this store a few times a year, a membership fee could very well outweigh the savings.

Next, ask about the interest rate. If the card offers special financing, find out how long you'll have to pay your balance before the regular rate kicks in, says Allan Katz, a New York-based CFP.

Related: Venmo now lets you shop online

You should also ask where you can use the card.

Some retailers will allow you to charge the account anywhere, just like you would a traditional Visa or Mastercard, while others limit you to store purchases.

Stand your ground

If you've decided that a store card isn't right for you, stay strong and don't let the sales associate persuade you.

"Their job is to get you to sign up," says Chris Chen, a Massachusetts-based CFP.

Once you've opened an account, it can be challenging to undo.

Related: Don't get duped by these online shopping scams

Canceling a credit card can hurt your credit score, especially if the account offered a substantial line of credit or was open for over 6 months.

That's because your credit score is affected by the number of accounts you have open and your total amount of available credit.