Currency traders may not be fast enough, at least when it comes to cleaning up their image.

At issue is a controversial custom called “last look,” which allows market makers to back out of a trade. After allegations of abuse, most major banks have recently taken steps to publicly disclose their trading standards to clients. But not all are on board. Top 10 dealer BNP Paribas doesn't. And a big player in high-frequency trading called Tower Research Capital says it doesn't need to.

Industry executives — still reeling from paying billions in currency-rigging fines — concede that the largely unregulated $5.1 trillion-a-day foreign-exchange market could use a dose of transparency. As part of an effort to police themselves, traders are hoping a global code of conduct that's due in May will help shore up their reputation before regulators crack down harder.