In my experience you don't need to enter your credit card info while signing up, but your ads won't start running until you enter payment info. You might also read this : https://adwords.google.com/support/bin/answer.py?answer=6390
they charge the signup fee of $5, unless your paying for that out of vouchers. Other than that, they take .75c as a check on the credit card, but that gets refunded after about 2 weeks.