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Its believed for most the FICO scoring gods like to see between 1% and 9% on your credit cards. So, I would pay off the amount to get you in that range (and you may have to play around with it each month to see what's best for your score). Then once the statement hits showing that small balance, I would make a another payment to bring it to zero and then repeat. You may want to use both cards at least once every 3 months or so, to make sure it doesn't get deactivated because of inactivity.
Oh, and are in you the credit steps program for each card? If so, I think one of the requirements is that you use the card at least once during the month in order to get your increases. So, maybe do a small purchase on each card and pay it off immediately.
You should always PIF to avoid intrest charges. If you want a small balance to report, then make a payment before the statement drop date. If you don;t know the statement drop date, then make a parent online as soon as a balance shows up. Most CCCs won;t let you make a payment unitl there is a balance and some may even limit the number of online payments you can make over time.