From the slides(Page 32)
Ideal rendering order:
- Opaque first, then alpha test
Why is it? Provided that I have an almost flat terrain and lots of grasses on it, rendering grasses before terrain can take the advantage of Early Z to cull a lot of pixels of terrain to shade.
Scene with Transparent polygons should be rendered in Z-sorted order from the most far to the closest (in respect to camera view direction).
Alpha channel is used usually for modulating transparency strength in which case your statement is not true as alpha test polygons may be transparent too.
In some cases is alpha channel used as a mask (for sprites or fonts for example) and rendered polygons are not transparent at all. In that case the statement is true (up to a point which depends on your scene organization and rendering pipeline).
For more info see
