Dithering question

I'm generating a grayscale 1 byte per pixel bitmap with Anti Grain.
I need to convert this to a black and white image for output on a thermal device.

How can I accomplish dithering with Anti Grain so that grayscale images look ok on black and white?
Also should I do the dithering on the entire bitmap once its generated or do it shape by shape as they are being drawn?

Thanks for the help.