Drop-down shower floor prep
I would probably still put a liner in.  It doesn't hurt to have, and a liner makes sure all the water goes where you want it to go.  You can use RedGuard in place of a shower pan liner, but make sure you read the product instructions on how to achieve the correct thickness.  I believe it's 2 coats of product to meet the requirements for a pan liner.

You can also mortar/tile directly over RedGuard (in fact, you have to - RedGuard is not meant to be left as a finished surface).