Are credit cards automatically saved for subscription orders, or does the customer have to elect to save their credit card at checkout?
The credit card is automatically saved for the subscription. It is not automatically saved to the account. The same credit card will be charged again when the recurring order is generated. The customer can log in and change it him/herself at any point. If the card fails, you have the option to automatically roll the purchase over to a saved credit card in the account if one exists. That is in Subscriptions / Preferences.