A quick and useful WebmasterWorld informs us that if your credit card is declines by Google, your AdWords quality score will not be hurt.
In fact, AdWordsAdvisor2 replies to the forum post saying:
Being suspended for a credit card decline will not effect your quality score. Your ad will have the same value in the auction upon your return as it did prior to the decline.
It is pretty much like when you pause an AdWords campaign, your quality score should not be affected.
Forum discussion at WebmasterWorld.