This largely depends on the services you are delivering. Design should only be based on hourly if it's a salary situation. The creative process isn't the same

This largely depends on the services you are delivering. Design should only be based on hourly if it's a salary situation. The creative process isn't the same as typing out HTML / CSS. Hourly rates - incentive for productivity gains - this is a two way street. I base my fixed prices on my hourly rates (though I may raise these rates significantly depending on the client and the job) and try to estimate the hours needed. If bid good, then I will get my hourly rate. If I bid horrible then I'm either making more money or losing money. Of course I want to get the job done as fast as possible (without cutting corners) to get the best rate possible for the job, but I also have to keep an eye on deadlines and the start date for the next project. There are always more factors than just money to give you incentive for productivity gains