I'm in the confused stage as well. I've done lots of offers and had many that wanted credit card numbers. What do I do? Do I wait for the trial to expire or till I get paid? Do I cancel the offer right away or does the company only pay you after the trial period? I don't really want to keep doing these offers if I'm going to end up with a lot of stuff I don't really want.
Technically you do not have to wait until any trails expire to get credit (unless it says so on the actual offer listing page). For instance, any trials on the 'free offers' page can be canceled at any time (or whatever certain amount of trail days it states you are required to go through).
I don't suggest signing up and just canceling right away, though. They only want you to sign up for offers you're actually interested in, and canceling right away would obviously make it look like you weren't interested (and can possibly result in not receiving credit, or so I've heard). So check them out, wait a few days or so and see how you like the business. If you like it, great. If it ends up being something you're not looking for, you don't have to keep it.
So basically, just do ones you're interested in. Yep.