Hm. In spirit, what you're doing seems fine (as you are quoting the same price either way). However, when it actually comes down to giving them an invoice with inflated hours, that is completely unethical. Definitely a tough one. The unfortunate thing about billing for extra hours is that it makes you seem slow. It does sound like you've had enough experience to know how long a job will take so perhaps you could give quotes for the entire project without an hourly rate.