Four Reasons Why You Should Never Use Google AdWords’ Pre-Pay Option.

1. Pre-Pay Cannot Be Changed to Post-Pay
2. Pre-Pay Payment History Cannot Be Used to Establish Invoicing as a Payment Option
3. Once Funds Are Exhausted, Google May Not Send Notification
4. You May Owe the Pre-Pay Funds, Regardless of If You Spend the Budget

via Search Engine Journal.

I don't like to pay my campaigns in advance. Let's assume you are starting a new campaign for a low-traffic site and you can't estimate in advance how well the campaign will perform. You might decide to stop the campaign before reaching your initial expectation and it is worth to pay only for the clicks you actually received.

Why paying Google Adwords campaigns in advance?

I advise you to select the postpay method and pay attention to your credit card balance to prevent unexpected block of campaign caused by the lack of funds.