(Just using my credit card as an score booster.....)

When I owe $50 on the bill, is it better to:

a.) Pay it all at once?
b.) Pay ALMOST all of it? Like, leaving $15 on there. And continuing that cycle month-after-month

I heard that option b. shows that you're not afraid to use your credit & that your responsible doing it the same time. Also, that creditors are always looking to throw in "rewards" for their card-holders.

Your score will be the highest if you have 7-10% usage on your total available credit.
Paying down to zero will actually make your score slightly lower than if you leave 7% outstanding.

Go ahead and pay it down like Banker says, then pull your report. The next month do it the way I said and compare. Then you will see what theory is "nonsense".
Good credit scores are one's that have large amounts of available credit but very small amounts of used credit (30%) . Late payments are credit score killers, that only time can fix (35%). How long you have had credit can only be acheived with time, but is a less percentage of the credit score equation (15%). Inquiries count for about 10% of the score. Different types of credit accounts ie: revolving, installment, mortgage, line of credit...etc.. 10%.