How model-specific are the screws among iPhones & between other Mnfrs?

Other than the obvious head pattern, and screw length, how model-specific are the screws in the pre-counted screw sets? I know that there are thread, shank, diameter, hardness and other differences among smart phone screws. But I'm wondering if it might be more practical to buy 20 or so of each screw type instead of a specific kit. That way, if you lose one or two, you don't have to wait a week or two for new screws to be delivered. I'm waiting on a back order of an Iphone 3g screw set for 3 weeks now because I dropped 2 screws.

And does anyone know a good supplier of bulk computer and smart phone screws?