The grace period
Paying the full statement balance by the due date normally means no interest on purchases. Carrying a balance can suspend that protection, so new purchases start accruing interest immediately until the balance is cleared in full again.
Utilisation
Utilisation is the reported balance divided by the limit. Issuers usually report the statement balance, so paying before the statement closes lowers the figure that reaches the bureaus, even if you already pay in full every month.
Do the rewards arithmetic
Estimate your annual spending in the categories the card rewards, apply the earn rate, then subtract the annual fee. Value only exists if the result is positive and you never pay interest on a carried balance.
Avoid cash advances
Cash advances normally carry a fee, a higher rate and no interest-free period. They are among the most expensive ways to use a card.