making objects appear/disappear in layout
 

what is the preferred way of making an object appear in a scene at a particular point in time. is the object always in the scene, off camera somewhere, and then gets keyed in at a particular time, or should the object be at 100% dissolved to start with, and then key the dissolve parameter, or is there a way to key when an object appears in the scene with the scene or graph editors?

It all depends on how you are wanting the object to appear.

If it's the case that the object is supposed to be visible in the scene at a point, but is off camera, then leave it where it is and animate it into the viewport so it still affects light bounces, etc.

If it is the case that the object doesn't exist anywhere until a particular point in time, then use an object dissolve.