Or only where the ceramic tile floor will be installed. Should the pan just sit on the 5/8 wood subfloor?
You can place the pan over the subfloor. You need to set the pan in mortar.
Won't hurt to put the durock down. Are you using a rubber drip pan, or steel? I like the rubber, much more versatile. I also like to put cement down, and set the base in while the cement is wet. This gives yo a good base, the fiberglass can't move and deteriorate.
Definitely use the cement board. 5/8 Plywood is really crappy subfloor. It just has too much give. The minimum should be 3/4 and 1 1/8 is ideal for a floor that won't dip, sag or cause tile to crack.
