Pro-rating is fine, just make it a month+. So you support with one week left in the billing cycle, they get charged for 5 weeks. You could make it an option that creators set.

I'd really like to see a debit system: I could load up 6 months worth of pledges at once and incur only one fee (whether it's explicitly charged or done 'hidden', I don't care). Creators could choose how often to be paid, as well, so they control what kind of fees they incur as well.