Standard disclaimer: invasive species are bad, mmkay. But, mandatory stickering of whitewater craft will not stop the spread of invasive mussels.
That said, here in Idaho, the program is mostly targeted at motor boats. In north Idaho there are billboards showing boat propellers covered with zebra mussels. Enforcement consists of voluntary check/wash stations around CDA and PDR. The feared enforcement at the popular whitewater put-ins and take outs has not materialized. At worst I heard that boats were being checked for stickers at the put ins for the permit runs on the Salmon, Selway, and Snake. But, the stickers are not in the Forest Service's jurisdiction, so its hard to say what is really going on in terms of enforcement on whitewater craft. There's also no telling how much revenue whitewater boaters pumped into the state's coffers as a result of the sticker mandate.
One thing's for sure: they weren't checking at any of the fine rivers I paddled throughout the state last season.
"A river is more than an amenity, it is a treasure."
-Oliver Wendell Holmes writing for the majority in New Jersey v. New York