It would depend on company policy. If company policy requires you to take PTO for a doctor's or dentists appointment, then I'd only work 40 hours at that company. If the company doesn't account for exempt time, then they may get more than 40 hours of work out of me.

Regarding exempt employees having to take PTO, recent court challenges in California have come out in favor of the employer. See Rhea V. General Atomics. http://caselaw.findlaw.com/ca-court-of-appeal/1673316.html