Yeah, you can do that or put all objects with the same texture on their own layer, so even accidental send to front/send to back won't change the layer, and still allows the GPU to keep the same texture loaded.
However, that's probably of the more subtle areas you could optimize. The canvas thing is much more important (don't make them bigger than the screen if you can). I wouldn't bother building your game from the ground up with the Z ordering thing in mind. It won't make any noticable difference unless you're using hundreds, or thousands, of on-screen objects which are intermingled with other objects in the Z order, which is a fairly rare situation.
You're much better off optimising the use of pixel shaders, or not bothering at all, until your framerates prove you actually need to start speeding things up (what's the point if it runs fine anyway?).