FALSE FALSE FALSE!!! The teller, who doesn't understand how credit cards work, is wanting their bank to make money of the interest you pay. You get no benefit from carrying a balance. If it's a high balance, you're in serious trouble if you need to get a loan. You'll save interest charges if you pay off your balance every month.
Your credit score, in part, is based upon how much you owe vs how much you could owe. It's called Credit Utilization Ratio. If your credit card has a balance of $500 and your credit limit is $2500, your ratio is 20%. Land mark ratios are 10%, 30% and >30%.
All other things being equal, those with excellent credit have ratios below 10%. Good credit runs between 10% and 30%. Fair to poor credit is above 30%. The more above 30%, the worse it is.
Your ratio is reported as of the statement closing date. If your $2500 card has a balance of $2000, but you pay it off just before the payment due date, your ratio has already been reported as 80% (very bad). You want to monitor your balance and pay it down to below $200 or pay it off in full if at all possible BEFORE the next statement closing date. Then your ratio will be in the excellent range.
You really don't need to use your card at all. That will keep your ratio to 0% (excellent). But use in once in a while to show that you are still a walking, talking human who hasn't died and assumed room temperature. They don't want to keep active accounts on dead people.