Skilled roadies will push a harder gear on rough sections of road. Pushing harder will put more weight on the pedals and less on your ass. This is worthless for fixies and singlespeeds though becuase you are commited to one gear. Don't worry about your bike on rough pavement...it can take it. Just do what feels most comfortable to you...this will probably involve taking weight off the saddle and, like Aeroplane said, avoid the grip of death on the bars. WHen I approad a bump at speed, I usually relax my grip on the bars and just let the bike rattle underneath me.