I tell my clients I spend their money the way I would spend my own. I am careful about not incurring unnecessary costs, but also, if for instance I have a $50/day food limit, if I spend $10 total one day on a fast food lunch and dinner, I don't want to be hassled about spending $60 the next day.

Then again, I've been lucky enough to work at companies with mostly reasonable travel policies, including averaging costs across the days of the trip.