I'd never tip for the sake of tipping, have you seen their hourly rate?
But punting is a selfish pastime and if I genuinely thought an extra £10 would get me a better service, then or subsequently, then I'd consider it. Hasn't happened to date. It's a bit of a catch-22 though, as if they gave a good enough service that I'd return then they're already, by definition, a good provider and probably don't need a tip to provide a good service next time.