Ask HN: When to ask for credit card number?

We are putting the finishing touches on our first web application, and I am curious as to what everyone's opinions are on when we should capture our user's credit card data.

Our current plan is to offer a 15 day free trial of our product. We were not planning on getting the user's card information during initial sign up. Our rationale for this was that we wanted to make the initial sign up process as easy as possible. I also assumed that it would reduce the likelihood of chargebacks.

Once the user signed up for their account, they would be able to use the application and save their card information in the application at their convenience. If, after the 15 day trial, they still had not entered their card information, their installation of the application would become "locked" until they entered their card info.

Are there good reasons why this is an inadvisable strategy, and why we would want to get the card information before the trial period begins?

Thanks in advance for your input.