By Richard Horgan Los Angeles (Knewz) — This may be the most useful article you read today. What’s the annual percentage rate on your go-to credit card? When’s the last time, if ever, that you called the issuing company to request a lower rate? According to LendingTree’s latest annual survey of credit cardholders, only 15 percent of respondents asked card issuers in the past year for a lower APR. Even though, in 70 percent of those cases, the results were successful, with on average a rate reduction of 6.9 percentage points. The consumer payoff was even more dramatic in other categories. Some …