For cycling purposes this really was answered with the first response 5 years ago. Even taking Soloist's extreme example of a 28% grade as an example the change between using the the hypotenuse and the flat leg as your denominator changes the result by a little over 1%. At 28% it won't help you. And for more common (i.e. lesser) pitches the difference will be less. At 10% the difference is .05%
Hey, let's have the math dorks really whip it out by calculating pi to the nth decimal, where n=stupid.