Okay sometimes I need things spelled out for me. So about this whole z ordering business let me see if I've got this straight.
Let's say within a certain layer of a layout you place all of your decorative scenery. Let's say you've got one tile for dirt, one for grass and one for clouds and you just use multiple copies of those three tiles to decorate the whole level. So if understand this correctly the z ordering means that you would want to say select all of your dirt tiles and send them all to the back. Then select all the grass tiles and send them all to the front. Then select all the cloud tiles and send them all to the front.
Is that what you guys mean by z ordering?
Yes, pretty much. You can also do it at runtime by making, say, a z-sort group that runs once at start of layout so you don't have to do it manually.
But this is pretty much just an optimization for lower-end cards if there are lots and lots of tiles, and honestly it's probably not going to give you a huge performance boost on it's own... maybe just a few more fps. But if you're optimizing a large game that's lagging then every frame per second counts, and shaving off a few here and there adds up.