we have tons of those in Seattle and I have never heard of any motorcycle, scooter or moped crashing on a bridge due to the grating in the 6 years I've lived hear. I cross twice every day and have never had any problems.
I find the slower you go the more 'wiggly' it feels. Just haul ass and don't steer too stiff, that's how we all do it every time. Even when they're wet I haul ass, they're no big deal, just feels a bit wierd.