Dry-set or modifeid thinset under backerboard

When applying a thinset mortar underneathe backerboard, does it matter if the thinset is a dry-set mortar or a does if have to be modified? Does it depend on the manufacturer as well?

Manufacturers make varying recommendations on that subject. The TCNA says unmodified is best but they also say follow the manufacturers recommendations.

[Edited by Bud Cline Tile on Mon, Nov 2, 2009 at 4:41 pm CST]