[galfarragem]

While a freelancer architect I will always bill value based and fixed-fee. IMO the extra costs that you might have to support are less relevant then the peace of mind of having clear numbers since the beggining. The client knows how much he will pay and will not be suspicious of your billing.

By the other hand, for a programmer (I have much less experience here), billing value based and fixed-fee is more risky. The specs are not so clear as in the architectural field and even (apparently) small changes that don't add a lot (if any) value for the client can multiply the time that you spend in a project.