I can find no other references to Riker's alleged exceptional skill as a pilot. However he does have prior experience as a pilot, having served as conn officer aboard the USS Pegasus.
Riker is more often noted as being an exceptional tactician as opposed to a pilot. From TNG: "Peak Performance":
DATA: I have several examples of Commander Riker's battle technique. At the Academy, he calculated a sensory blind spot on a Tholian vessel and hid within it during a battle simulation. And as a lieutenant aboard the Potemkin, his solution to a crisis was to shut down all power, and hang over a planet's magnetic poles, thus confusing his opponent's sensors.
Only twenty-one percent of the time does he rely upon traditional tactics. So, the Captain must be prepared for unusual cunning.
Hiding within a Tholian vessel's sensor blindspot might be an indication of piloting finesse, depending on the size of said blindspot. But I admit it's a serious stretch.
Furthermore, there is some evidence before season 6 that directly supports the conclusion that Riker is not the best pilot on the Enterprise.
There are at least two occasions on which the Enterprise required a superb pilot in order to safely navigate a dangerous situation. In "Booby Trap" (season 3) the Enterprise must navigate out of an asteroid field littered with dangerous radiation fields. Simulations show that the ship's computer isn't good enough to reliably make it out without the crew dying of radiation exposure, and Geordi concludes that only a human pilot could get them out safely. Despite the enormous risk, it is Picard that takes the conn, not Riker.
In "In Theory" (season 4) the Enterprise is stuck in a nebula littered with dark matter pockets that produce dangerous spatial distortions when they collide with the ship. They decide to use a shuttle craft to scope a safe path out of the Nebula, since its small size lets it change direction more responsively. Once again, it is Picard that pilots the shuttlecraft. Riker objects due to safety concerns, but no one ever asks who's actually the better pilot.
PICARD: I believe our best chance of escaping this situation is for me to pilot the shuttle. It's my ship, Will. I've got to do this.