It's not easy to get, say, chlamydia via receiving cunnilingus. It's also not true that if they have a case in their urethra, they're likely to have one in their throat. They might, but as it usually doesn't have any symptoms, no-one can be sure about the incidence.
But, as ever, it depends on your attitude to risk and how valuable what you're considering giving up is.
There are plenty of clients who are fine with OW. I suspect fewer will be interested in covered RO.