This question already has an answer here:
Until recently, I was under the impression that it's bad for your credit history to not pay all bills on time and in full.
However, a banker recently told me that it's a good idea to carry a small balance on credit cards even if you have the means to pay them off in full each month. In her words, "it doesn't look good when you use credit cards as cash equivalents." She recommended leaving $40 or $50 unpaid on each bill. She also mentioned that the exact amount to leave depended on the maximum credit line of the card, where the higher the credit limit, the more you should leave unpaid.
myfico.com somewhat supports that claim:
In some cases, having a very small balance without missing a payment shows that you have managed credit responsibly, and may be slightly better than carrying no balance at all.
A search of this site turned up a recommendation to carry a $10 balance from month to month.
The obvious downside to carrying a balance is that you have to pay interest the following month. (If there are other downsides I haven't thought of, I'd appreciate hearing about them.)
What factors should I look at in my own financial situation to determine whether carrying a small balance on my credit cards would help me, and if it would, how do I figure out how much to carry?