[macguyver]

From first hand experience helping startups:

If your app has the remote possibility that it can be abused by fraudsters, scammers, con people, ask for credit card at the start of the trial. This will save you a lot of hassle later down the road.

Otherwise, you can ask at the end.

Keep in mind that a qualified user will sign up for your service even if you ask for the CC up front. As a business owner, your job is to get users, yes, but also to get paid for your efforts. Hopefully this will help you decide what to do. Remember, it's much easier if you just treat it like a learning experience.